1. science
2. ecology
3. pollution

VOLUME II: CHAPTER 12
HOW TO INCORPORATE THE
EFFECTS OF AIR POLLUTION
CONTROL DEVICE EFFICIENCIES
AND MALFUNCTIONS INTO EMISSION
INVENTORY ESTIMATES
July 2000
Prepared by:
Eastern Research Group, Inc.
Prepared for:
Point Sources Committee
Emission Inventory Improvement Program
DISCLAIMER
This document was furnished to the Emission Inventory Improvement Program and the U.S.
Environmental Protection Agency by Eastern Research Group, Inc., Morrisville, North Carolina.
This report is intended to be a working draft document and has not been reviewed and approved
for publication. The opinions, findings, and conclusions expressed represent a consensus of the
members of the Emission Inventory Improvement Program.
ACKNOWLEDGMENT
This document was prepared by Eastern Research Group, Inc., for the Point Sources Committee
of the Emission Inventory Improvement Program and for Roy Huntley of the Emission Factor
and Inventory Group, U.S. Environmental Protection Agency. Members of the Point Sources
Committee contributing to the preparation of this document are:
Denise Alston-Guiden, Galson Consulting
Lynn Barnes, South Carolina Department of Health and Environmental Control
Bob Betterton, Co-Chair, South Carolina Department of Health and Environmental Control
Paul Brochi, Texas Natural Resource Conservation Commission
Richard Forbes, Illinois Environmental Protection Agency
Alice Fredlund, Louisiana Department of Environmental Quality
Marty Hochhauser, Allegheny County Health Department
Roy Huntley, Co-Chair, Emission Factor and Inventory Group, U.S. Environmental Protection Agency
Toch Mangat, Bay Area Air Quality Management District
Ralph Patterson, Wisconsin Department of Natural Resources
Anne Pope, Emission Factor and Inventory Group, U.S. Environmental Protection Agency
Jim Southerland, North Carolina Department of Environment and Natural Resources
Bob Wooten, North Carolina Department of Environment and Natural Resources
EIIP Volume II
iii
Draft 7/14/00
CHAPTER 12 - CONTROL DEVICES
This page is intentionally left blank
iv
EIIP Volume II
CONTENTS
SECTION
1
2
PAGE
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1-1
1.1
What is Control Efficiency? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1-3
1.2
How Do I Determine Control Efficiency? . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1-3
1.3
Can I Assume APCDs are Always Operated at the Maximum Level of
Efficiency? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1-5
1.4
How Do I Estimate Actual Control Efficiency? . . . . . . . . . . . . . . . . . . . . . 12.1-5
1.5
When Multiple Control Devices are Used, are Their Efficiencies Additive? 12.1-6
Description of Criteria Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-1
2.1
Ozone (O3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-1
2.2
Nitrogen Oxides (NOx) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 How are Nitrogen Oxides Formed in Stationary Combustion
Sources? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 How are Nitrogen Oxides Formed in Stationary Noncombustion
Sources? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 What Characteristics of Nitrogen Oxides Determine the Type of
Air Pollution Control Device Used for Emissions Control? . . . . . .
12.2-1
12.2-1
12.2-3
12.2-3
2.3
Sulfur Dioxide (SO2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-4
2.3.1 How is Sulfur Dioxide Formed? . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-4
2.3.2 What Characteristics of Sulfur Dioxide Determine the Type of Air
Pollution Control Device Used for Emissions Control? . . . . . . . . . 12.2-4
2.4
Volatile Organic Compounds (VOC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-5
2.4.1 How are Volatile Organic Compounds Formed? . . . . . . . . . . . . . . . 12.2-5
2.4.2 What Characteristics of Volatile Organic Compounds Determine
the Type of Air Pollution Control Device Used for Emissions
Control? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-5
2.5
Particulate Matter (PM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-5
2.5.1 How is Particulate Matter Formed? . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-5
EIIP Volume II
v
CONTENTS (CONTINUED)
SECTION
PAGE
2.5.2 What Characteristics of Particulate Matter Determine the Type of
Air Pollution Control Device Used for Emissions Control? . . . . . . 12.2-6
2.6
Carbon Monoxide (CO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-7
2.6.1 How is Carbon Monoxide Formed? . . . . . . . . . . . . . . . . . . . . . . . . . 12.2-7
2.6.2 What Characteristics of Carbon Monoxide Determine the Type of
Air Pollution Control Device Used for Emissions Control? . . . . . . 12.2-7
3
Control Techniques for Criteria Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-1
vi
3.1
How Are Appropriate Air Pollution Control Devices Selected? . . . . 12.3-1
3.2
Control of Nitrogen Oxides Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 What Process Air Pollution Control Devices are Typically Used
to Control Nitrogen Oxides Emissions? . . . . . . . . . . . . . . . . . . . . .
3.2.2 What Combustion Air Pollution Control Devices are Typically
Used to Control Nitrogen Oxides Emissions? . . . . . . . . . . . . . . . . .
3.2.3 What Post-process Air Pollution Control Devices are Typically
Used to Control Nitrogen Oxides Emissions? . . . . . . . . . . . . . . . .
12.3-3
12.3-3
12.3-3
12.3-4
3.3
Control of Sulfur Dioxide Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-4
3.3.1 What Process Air Pollution Control Devices are Typically Used to
Control Sulfur Dioxide Emissions? . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-4
3.3.2 What Post-process Air Pollution Control Devices are Typically
Used to Control Sulfur Dioxide Emissions? . . . . . . . . . . . . . . . . . . 12.3-6
3.4
Control of Volatile Organic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-6
3.4.1 What Process Air Pollution Control Devices are Typically Used to
Control Volatile Organic Compounds Emissions? . . . . . . . . . . . . . 12.3-6
3.4.2 What Post-process Air Pollution Control Devices are Typically
Used to Control Volatile Organic Compounds Emissions? . . . . . . . 12.3-6
3.5
Control of Particulate Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-7
3.5.1 What Process Air Pollution Control Devices are Typically Used to
Control Particulate Matter Emissions? . . . . . . . . . . . . . . . . . . . . . . 12.3-7
3.5.2 What Post-process Air Pollution Control Devices are Typically
Used to Control Particulate Matter Emissions? . . . . . . . . . . . . . . . . 12.3-8
EIIP Volume II
CONTENTS (CONTINUED)
SECTION
3.6
4
PAGE
Control of Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-9
3.6.1 What Process Air Pollution Control Devices are Typically Used to
Control Carbon Monoxide Emissions? . . . . . . . . . . . . . . . . . . . . . . 12.3-9
3.6.2 What Post-process Combustion Air Pollution Control Devices are
Typically Used to Control Carbon Monoxide Emissions? . . . . . . 12.3-10
Descriptions of Air Pollution Control Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-1
4.1
4.2
Selective Catalytic Reduction (SCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 What Pollutants are Controlled Using Selective Catalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 How Does Selective Catalytic Reduction Work? . . . . . . . . . . . . . .
4.1.3 What Reducing Agent is Used in Selective Catalytic Reduction? . .
4.1.4 What Catalysts are Used in Selective Catalytic Reduction? . . . . . .
4.1.5 What Issues are of Concern When Using Selective Catalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.6 What Wastes Result From Using Selective Catalytic Reduction? . .
12.4-1
12.4-1
12.4-1
12.4-2
12.4-3
12.4-4
12.4-4
Selective Noncatalytic Reduction (SNCR) . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 What Pollutants are Controlled Using Selective Noncatalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 How Does Selective Noncatalytic Reduction Work? . . . . . . . . . . .
4.2.3 What Reducing Agents are Used in Selective Noncatalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.4 What Issues are of Concern When Using Selective Noncatalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.5 What Wastes Result From Using Selective Noncatalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.4-5
4.3
Low NOx Burners (LNB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 What Pollutants are Controlled Using Low NOx Burners? . . . . . . .
4.3.2 How Do Low NOx Burners Work? . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3 What Issues are of Concern When Using Low NOx Burners? . . . . .
4.3.4 What Wastes Result From Using Low NOx Burners? . . . . . . . . . . .
12.4-6
12.4-6
12.4-6
12.4-8
12.4-8
4.4
Natural Gas Burner/Reburn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-8
4.4.1 What Pollutants are Controlled Using Natural Gas Burner/Reburn? 12.4-8
4.4.2 How Does Natural Gas Burner/Reburn Work? . . . . . . . . . . . . . . . . 12.4-8
EIIP Volume II
12.4-5
12.4-5
12.4-6
12.4-6
12.4-6
vii
CONTENTS (CONTINUED)
SECTION
PAGE
4.4.3 What Issues Are of Concern When Using Natural Gas
Burner/Reburn? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-10
4.4.4 What Wastes Result From Using Natural Gas Burner/Reburn? . . 12.4-10
viii
4.5
Water/Steam Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.1 What Pollutants are Controlled Using Water/Steam Injection? . .
4.5.2 How Does Water /Steam Injection Work? . . . . . . . . . . . . . . . . . .
4.5.3 What Issues are of Concern When Using Water /Steam Injection?
4.5.4 What Wastes Result From Using Water/Stream injection? . . . . . .
12.4-11
12.4-11
12.4-11
12.4-11
12.4-11
4.6
Staged Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.1 What Pollutants are Controlled Using Staged Combustion? . . . . .
4.6.2 How Does Staged Combustion Work? . . . . . . . . . . . . . . . . . . . . .
4.6.3 What Issues are of Concern When Using Staged Combustion? . .
4.6.4 What Waste Results from Using Staged Combustion? . . . . . . . . .
12.4-11
12.4-11
12.4-11
12.4-13
12.4-13
4.7
Flue Gas Recirculation (FGR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.1 What Pollutants are Controlled Using Flue Gas Recirculation? . .
4.7.2 How Does Flue Gas Recirculation Work? . . . . . . . . . . . . . . . . . . .
4.7.3 What Issues are of Concern When Using Flue Gas Recirculation?
4.7.4 What Wastes Result from Using Flue Gas Recirculation? . . . . . .
12.4-14
12.4-14
12.4-14
12.4-14
12.4-14
4.8
Low Excess Air (LEA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.1 What Pollutants are Controlled Using Low Excess Air? . . . . . . . .
4.8.2 How Does Low Excess Air Work? . . . . . . . . . . . . . . . . . . . . . . . .
4.8.3 What Issues are of Concern When Using Low Excess Air? . . . . .
4.8.4 What Wastes Result from Using Low Excess Air? . . . . . . . . . . . .
12.4-15
12.4-15
12.4-15
12.4-15
12.4-15
4.9
Staged Overfire Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9.1 What Pollutants are Controlled Using Staged Overfire Air? . . . . .
4.9.2 How Does Staged Overfire Air Work? . . . . . . . . . . . . . . . . . . . . .
4.9.3 What Issues are of Concern When Using Staged Overfire Air? . .
4.9.4 What Wastes Result From Using Staged Overfire Air? . . . . . . . .
12.4-15
12.4-15
12.4-16
12.4-16
12.4-16
4.10
Nonselective Catalytic Reduction (NSCR) . . . . . . . . . . . . . . . . . . . . . . . . 12.4-17
4.10.1 What Pollutants are Controlled using Nonselective Catalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-17
4.10.2 How Does Nonselective Catalytic Reduction Work? . . . . . . . . . . 12.4-17
EIIP Volume II
CONTENTS (CONTINUED)
SECTION
PAGE
4.10.3 What Issues are of Concern When Using Nonselective Catalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-19
4.10.4 What Wastes Result from Using Nonselective Catalytic
Reduction? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-19
4.11
Wet Acid Gas Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.1 What Pollutants are Controlled Using Wet Acid Gas Scrubbers ?
4.11.2 How Do Wet Acid Gas Scrubbers Work? . . . . . . . . . . . . . . . . . . .
4.11.3 What Sorbent Material is Used in Wet Acid Gas Scrubbers? . . . .
4.11.4 What Issues are of Concern When Using Wet Acid Gas
Scrubbers? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.5 What Wastes Result from Using Wet Acid Gas Scrubbers ? . . . .
12.4-20
12.4-20
12.4-20
12.4-20
Spray Dryer Absorbers (SDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.12.1 What Pollutants are Controlled Using Spray Dryer Absorbers? . .
4.12.2 How Do Spray Dryer Absorbers Work? . . . . . . . . . . . . . . . . . . . .
4.12.3 What Sorbent Material is Used in Spray Dryer Absorbers? . . . . .
4.12.4 What Issues are of Concern When Using Spray
Dryer Absorbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.12.5 What Wastes Result from Using Spray Dryer Absorbers? . . . . . .
12.4-22
12.4-22
12.4-22
12.4-23
4.13
Dry Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13.1 What Pollutants are Controlled Using Dry Injection? . . . . . . . . . .
4.13.2 How Does Dry Injection Work? . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13.3 What Sorbent Material is Used In Dry Injection? . . . . . . . . . . . . .
4.13.4 What Issues are of Concern When Using Dry Injection? . . . . . . .
4.13.5 What Wastes Result from Using Dry Injection? . . . . . . . . . . . . . .
12.4-24
12.4-24
12.4-24
12.4-25
12.4-25
12.4-25
4.14
Carbon Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.14.1 What Pollutants are Controlled Using Carbon Adsorption? . . . . .
4.14.2 How Does Carbon Adsorption Work? . . . . . . . . . . . . . . . . . . . . . .
4.14.3 What Sorbent Material is Used in Carbon Adsorption? . . . . . . . .
4.14.4 What Issues are of Concern When Using Carbon Adsorption? . . .
4.14.5 What Wastes Result from Using Carbon Adsorption? . . . . . . . . .
12.4-25
12.4-25
12.4-25
12.4-26
12.4-27
12.4-27
4.15
Thermal Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-27
4.15.1 What Pollutants are Controlled Using Thermal Oxidation? . . . . . 12.4-27
4.15.2 How Does Thermal Oxidation Work? . . . . . . . . . . . . . . . . . . . . . . 12.4-27
EIIP Volume II
ix
4.12
12.4-20
12.4-22
12.4-23
12.4-24
CONTENTS (CONTINUED)
SECTION
PAGE
4.15.3 What Issues are of Concern When Using Thermal Oxidation? . . . 12.4-28
4.15.4 What Wastes Result from Using Thermal Oxidation? . . . . . . . . . . 12.4-28
4.16
Catalytic Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.16.1 What Pollutants are Controlled Using Catalytic Oxidation? . . . . .
4.16.2 How Does Catalytic Oxidation Work? . . . . . . . . . . . . . . . . . . . . .
4.16.3 What Catalyst Material is Used in Catalytic Oxidation? . . . . . . . .
4.16.4 What Issues are of Concern When Using Catalytic Oxidation? . .
4.16.5 What Wastes Result from Using Catalytic Oxidation? . . . . . . . . .
12.4-28
12.4-28
12.4-29
12.4-29
12.4-30
12.4-30
4.17
Flares
4.17.1
4.17.2
4.17.3
4.17.4
.....................................................
What Pollutants are Controlled Using Flares? . . . . . . . . . . . . . . . .
How Do Flares Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What Issues are of Concern When Using Flares? . . . . . . . . . . . . .
What Wastes Result from Using Flares? . . . . . . . . . . . . . . . . . . . .
12.4-31
12.4-31
12.4-31
12.4-31
12.4-33
4.18
Floating Roof Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.18.1 What Pollutants are Controlled Using Floating Roof Tank
Systems? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.18.2 How Do Floating Roof Tank Systems Work? . . . . . . . . . . . . . . . .
4.18.3 What Issues are of Concern When Using Floating Roof Tank
Systems? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.18.4 What Wastes Result from Using Floating Roof Tank Systems? . .
12.4-33
Mechanical Collectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.19.1 What Pollutants are Controlled Using Mechanical Collectors ? . .
4.19.2 How Do Mechanical Collectors Work? . . . . . . . . . . . . . . . . . . . . .
4.19.3 What Issues are of Concern When Using Mechanical
Collectors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.19.4 What Wastes Result from Using Mechanical Collectors? . . . . . . .
12.4-34
12.4-34
12.4-34
Electrostatic Precipitators (ESP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.20.1 What Pollutants are Controlled Using Electrostatic Precipitators?
4.20.2 How Do Electrostatic Precipitators Work? . . . . . . . . . . . . . . . . . .
4.20.3 What Issues are of Concern When Using Electrostatic
Precipitators? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.20.4 What Wastes Result from Using Electrostatic Precipitators? . . . .
12.4-38
12.4-38
12.4-38
4.19
4.20
x
12.4-33
12.4-33
12.4-34
12.4-34
12.4-36
12.4-36
12.4-41
12.4-42
EIIP Volume II
CONTENTS (CONTINUED)
SECTION
5
6
PAGE
4.21
Fabric Filters (FF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.21.1 What Pollutants are Controlled Using Fabric Filters? . . . . . . . . . .
4.21.2 How Do Fabric Filters Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.21.3 What Issues are of Concern When Using Fabric Filters? . . . . . . .
4.21.4 What Wastes Result from Using Fabric Filters? . . . . . . . . . . . . . .
12.4-42
12.4-42
12.4-42
12.4-45
12.4-48
4.22
Wet PM Scrubbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.22.1 What Pollutants are Controlled Using Wet PM Scrubbers? . . . . .
4.22.2 How Do Wet PM Scrubbers Work? . . . . . . . . . . . . . . . . . . . . . . .
4.22.3 What Issues are of Concern When Using Wet PM Scrubbers? . . .
4.22.4 What Wastes Result from Using Wet PM Scrubbers? . . . . . . . . .
12.4-48
12.4-48
12.4-48
12.4-51
12.4-53
4.23.
When Are Multitude Control Devices Used? . . . . . . . . . . . . . . . . . . . . . . 12.4-53
Effects of Air Pollution Control Device Malfunctions on Emissions . . . . . . . . . . . 12.5-1
5.1
Excess Emissions from Air Pollution Control Device Malfunctions . . . . . 12.5-1
5.1.1 What are Some Examples of Excess Emissions? . . . . . . . . . . . . . . 12.5-1
5.1.2 What are Some Specific Causes of Excess Emissions from Control
Device Malfunctions? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5-2
5.2
Impact of Excess Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Why are Malfunctioning Control Devices a Concern? . . . . . . . . . .
5.2.2 Why is it Important to Track These Emissions? . . . . . . . . . . . . . . .
5.2.3 How Do Excess Emissions from Air Pollution Control Device
Malfunctions Affect Emission Inventories? . . . . . . . . . . . . . . . . . .
12.5-3
12.5-3
12.5-3
12.5-3
5.3
Accounting for Excess Emissions in an Emission Inventory . . . . . . . . . . . 12.5-4
5.3.1 What is the Efficiency of the Control Device during Process Upset
Conditions? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5-4
5.3.2 How can Releases during Control Device malfunctions be
calculated? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5-4
5.3.3 For Those Cases in Which Excess Emissions from Control Device
Malfunctions Can Be Reasonably Estimated, How Can You Collect the
Relevant Data? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5-5
5.4
Conclusion and Comment Solicitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5-5
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6-1
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TABLES AND FIGURES
TABLES
PAGE
12.3-1
Control Efficiencies (%) for NOx by Source Category and Control
Device Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-11
12.3-2
Control Efficiencies (%) for SO2 by Source Category and Control
Device Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-15
12.3-3
Control Efficiencies (%) for VOC by Source Category and Control
Device Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-16
12.3-4
Potential PM10 Emission Reductions with Fuel Switching (%) . . . . . . . . . 12.3-24
12.3-5
Potential PM2.5 Emission Reductions with Fuel Switching (%) . . . . . . . . 12.3-24
12.3-6
Control Efficiencies (%) for PM by Source Category and Control
Device Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-25
12.3-7
Control Efficiencies (%) for CO by Source Category and Control
Device Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3-31
12.4-1
Temperature and Chemical Resistance of Some Common Industrial
Fabrics Used in Fabric Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-46
Figures
12.4-1
Removal of NOx by SCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-2
12.4-2
Schematic Flow Diagram for the Selective Catalytic Reduction Method
of NOx Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-3
12.4-3
Schematic Diagram of a Typical Reburn System . . . . . . . . . . . . . . . . . . . . 12.4-9
12.4-4
Schematic of a Nonselective Catalytic Reduction System Design with a
Single Calalytic Reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-18
12.4-5
Schematic Process Flow Diagram for a Limestone-based SO2 Wet
Scrubbing System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-21
12.4-6
Spray Dryer Absorber System Schematic . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-23
xii
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TABLES AND FIGURES (CONTINUED)
FIGURES
Page
12.4-7
Schematic of a Thermal Oxidizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-29
12.4-8
Schematic of Catalytic Oxidizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-30
12.4-9
Typical Open Flare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-32
12.4-10
Schematic Flow Diagram of a Standard Cyclone . . . . . . . . . . . . . . . . . . . 12.4-37
12.4-11
Cutaway View of an Electrostatic Precipitator . . . . . . . . . . . . . . . . . . . . . 12.4-39
12.4-12
Particle Charging and Collection Within an ESP . . . . . . . . . . . . . . . . . . 12.4-40
12.4-13
Fabric Filter Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-44
12.4-14
Schematic of How Wet PM Scrubbers Remove Particles . . . . . . . . . . . . 12.4-50
12.4-15
Tray- or Sieve-type Scrubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4-52
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xiv
EIIP Volume II
1
INTRODUCTION
There are two approaches used by owners and operators of industry to reduce pollution:
Preventing pollution from forming by alternating manufacturing or production
practices, substitution of raw materials, or improved process control methods; and
Achieving emissions reductions with control equipment which capture or destroy
pollutants which would otherwise be released.
Sometimes, both approaches are employed.
Emission inventory preparers often have to estimate emission reductions or emission control
efficiencies of specific types of air pollution control devices (APCDs). Also, they sometimes
must estimate the effect on emission levels caused by APCD malfunctions. Depending on the
known operating characteristics of the facility and purposes of the inventory, state and local
inventory preparers may need to apply an adjustment to the control device efficiency values to
correct the underestimation of emissions if the control efficiency used is based on design
specifications or is based on controls specified by a regulation. Applying an adjustment has the
effect of reducing the assumed control device efficiency and increasing the estimated emissions.
This is a reasonable assumption since control equipment may sometimes fail or be off line for
maintenance, etc. This adjustment has been incorrectly called Rule Effectiveness (RE) and a
default value of 80 percent has been frequently used. Much confusion has existed over the
Environmental Protection Agency’s (EPA) RE policy and its application. EPA has drafted a
paper that clarifies the confusion and addresses the applicability of RE to emission inventories
and the draft paper is included in Appendix A. Additionally, the Emission Inventory
Improvement Program (EIIP) Point Sources Committee has published a technical paper, titled
Emission Inventories and Proper Use of Rule Effectiveness, addressing the application of RE to
emission inventories. The technical paper is also included in Appendix B and may be accessed
through EPA’s CHIEF web site at: http://www.epa.gov/ttn/chief. Guidance contained in
Chapter 12 allows the inventory preparer to avoid the necessity of using the 80 percent
adjustment factor.
This document provides background information and can be used as a primer to gain a basic
understanding of different air pollution control devices, how they work, the pollutants they
control, and how to adjust emission estimates to account for APCD malfunction.
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This document focuses only on the following basic types of APCDs used in industry today:
Mechanical collectors;
Scrubbers;
Fabric filters;
Electrostatic precipitators;
Incinerators;
Condensers;
Catalytic reactors;
Absorbers (where pollutants are collected as the molecules pass through the
surface of the absorbent to become distributed throughout the phase); and
Adsorbers (where pollutants are collected by concentration on the surface of a
liquid or solid).
Various terms may be used to describe a specific air pollution control device. Appendix C
presents a cross-reference for terms used to identify air pollution control devices.
EPA uses “criteria pollutants” as indicators of air quality. These pollutants are:
12.1-2
Ozone (O3);
Nitrogen oxides (NOx);
Sulfur dioxide (SO2);
Particulate matter with aerodynamic diameter less than or equal to 10 microns
(PM10);
Particulate matter with aerodynamic diameter less than or equal to 2.5 microns
(PM2.5);
Carbon monoxide (CO); and
Lead (Pb).
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While industrial ozone emissions are not usually regulated, EPA also regulates emissions of
volatile organic compounds (VOC) under criteria pollutant programs. VOC are ozone
precursors—they react with NOx in the atmosphere in the presence of sunlight to form ozone.
This document identifies the APCDs used for criteria pollutants and presents ranges of typical
control efficiencies. Section 2 describes the criteria pollutants. Sections 3 and 4 of this
document discuss the different types of APCDs and the pollutants they are intended to control.
Appendix G presents data sources for information presented in Section 5. Appendix D provides
details on how the control efficiency data were compiled and presented in Section 3 of this
document and contains a summary of additional data acquired but not evaluated for this
document. Section 5 presents the information necessary (including example calculations) to
assist an inventory preparer in determining the impact of APCD malfunction on emissions from
point sources. Example calculations and example scenarios are presented in Appendices E and
F.
For detailed descriptions and additional information on APCDs, refer to the references listed in
Section 6 of this document. Appendix G presents data sources for information presented in
Section 5.
1.1 WHAT IS CONTROL EFFICIENCY?
Control efficiency (CE) is a measure of emission reduction efficiency. It is a percentage value
representing the amount of emissions that are controlled by a control device, process change, or
reformulation.
1.2 HOW DO I DETERMINE CONTROL EFFICIENCY?
Control efficiency is calculated as:
Uncontrolled Emission Rate Controlled Emission Rate
100
Uncontrolled Emission Rate
(12.1-1)
If the emission rates (or concentration) are not known and the control efficiency cannot be
calculated, another method for determining efficiency is to refer to Section 3 of this document
that presents summary tables (Tables 12.3-1 through 12.3-7) for the control efficiencies of
APCDs used to reduce nitrogen oxides, sulfur dioxide, volatile organic compounds, particulate
matter, and carbon monoxide. These values are averages and may not be accurate for individual
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situations. Consult permit applications for APCD design efficiencies of particular equipment if
needed. Refer to Section 5 for determining control efficiency during APCD malfunction.
For fabric filters, which are used to reduce PM emissions, if the actual (measured) concentration
of PM in the inlet stream to the fabric filter and the expected concentration of PM in the outlet
stream are known, Equation 12.1-1 may be used to back calculate the control efficiency.
Generally, fabric filters are designed to reduce overall PM emissions to below an expected
concentration when the inlet concentrations are within a specified range. For example, the
design specifications for a fabric filter may state that the expected outlet emissions are 0.01
grains of PM per dry standard cubic foot of stack gas (gr/dscf) when the inlet emissions are
between 5 and 20 gr/dscf. Thus, the outlet emission rate remains relatively “constant” even
though the inlet concentration varies and, as the inlet emissions decrease, the overall control
efficiency is decreased. Therefore controlled emissions are calculated using the dust loading in
the flue gas and the exhaust flow rate. There is no need to estimate the control efficiency.
Example 12.1-1 shows how PM emissions are calculated using stack gas outlet concentrations
and flow rate.
Example 12.1-1
This example shows how to estimate PM emissions from a fabric filter when exit gas flow
rate and dust concentration is known.
EPM= Q x C where:
Q = exit gas flow rate (dscf/min)
C = PM concentration (gr/dscf)
EPM
= QxC
= 50,000 (dscf/min) x 0.01 (gr/dscf)
= 500 (gr/min)
To convert to (lb/hr):
E(lb/hr)
= E (gr/min) x (1 lb/7,000 gr) x (60 min/hr)
= 500 (gr/min) x (1 lb/7,000 gr) x (60 min/hr)
= 4.29 (lb/min)
Note that in this example inlet concentration and control efficiency data are not needed.
12.1-4
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1.3 CAN I ASSUME APCDS ARE ALWAYS OPERATED AT THE
MAXIMUM LEVEL OF EFFICIENCY?
No. Some facilities do not always operate devices at their maximum level of efficiency.
Although APCDs should be designed to accommodate reasonable process variation and some
deterioration, some types of control devices vary in efficiency based on process equipment
operating rates, fuel quality, and age. Usually an emission limit must be met and the primary
goal of the facility is to meet that limit. It may or may not be necessary to operate the control
device at its maximum level of efficiency in order to meet that limit. Also, in most cases,
operation below maximum efficiency can reduce operating costs.
Moreover, as detailed in Section 1.4, there are many factors that may reduce the level of
efficiency of a control device.
1.4 HOW DO I ESTIMATE ACTUAL CONTROL EFFICIENCY?
You can use the control efficiencies presented in Tables 12.3-1 through 12.3-7 and information
about the operating conditions of the device to estimate actual control efficiency. These are
typical values. Facility operators, engineers, and maintenance personnel are most qualified to
provide more specific information. You can also use the references in each table for more
information. Permit applications may also provide information.
Questions that should be asked or information that should be obtained, are described below:
How old is the control device? Some devices are affected by age and their
control efficiencies deteriorate over time if not properly maintained. In the case
of an ESP, for example, the collection efficiency declines due to corrosion,
warpage, and the accumulation of non-removeable dust on surfaces.
Is the control device properly maintained? Most devices require routine
maintenance and some devices may require intensive maintenance. For example,
the bags (filters) in a fabric filter should be cleaned when they are blinded by a
permanently entrained cake of particulate matter. Bags can also develop rips if
not replaced frequently enough. The fields in an electrostatic precipitator must be
maintained to operate at a specific voltage. If a device is not properly maintained,
the control efficiency will be reduced.
Is the device operated under conditions necessary for maximum efficiency and are
these conditions monitored? A fabric filter may be designed to operate at a
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specific pressure drop in order to attain maximum efficiency and the pressure
should be monitored. A thermal incinerator must operate at a particular
temperature and residence time, and these parameters should also be monitored.
Wet scrubbers must have the scrubbing liquor available at all times in proper
amounts. When a device is not operated properly, the control efficiency will be
reduced.
What is the throughput to the control device relative to its design capacity? If a
device is operated above its design capacity, the control efficiency may be
reduced. For example, if too much gas is forced through a wet scrubber,
channeling of gas can result and the control efficiency is reduced.
Note: The inventory preparer should use the information obtained from facility personnel to
determine the adjustment to the control efficiency value provided in Tables 12.3-1 through
12.3-7 to estimate an actual control efficiency.
1.5 WHEN MULTIPLE CONTROL DEVICES ARE USED, ARE THEIR
EFFICIENCIES ADDITIVE?
No. In general, when estimating the overall control efficiency for a combination of control
devices in series, inventory preparers should not assume the overall efficiency is additive or
cumulative. This is because control efficiency for a particular device is often dependent on the
inlet concentration. The overall control efficiency of a series of APCDs is typically higher than
the efficiencies of the individual control devices, but smaller than the sum of the individual
control efficiencies. However, in some cases the control efficiencies of multiple devices in
series may be assumed to be additive. In this case, the overall control efficiency of a series of
"n" devices is:
CE (overall) = 1 - [ (1 - CE1/100) * (1 - CE2/100) * ............*(1 - CEn/100) ] (12.1-2)
When the last device in a series of control devices is a fabric filter, you should assume that the
control efficiency of the APCDs is equal to the control efficiency of the fabric filter, and the
other devices help to reduce the load on the fabric filter. For example, suppose a wood boiler is
equipped with a multicyclone designed to operate at a control efficiency of 60 percent and a
fabric filter designed to operate at 99 percent, then the overall control efficiency is likely to be
around 99 percent, and for all practical purposes, can be assumed to be 99 percent.
12.1-6
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2
DESCRIPTION OF CRITERIA
POLLUTANTS
2.1 OZONE (O3)
Ozone, a colorless gas, is the major component of smog. Except for very low levels of emissions
from a limited number of processes, ozone is not emitted directly into the air but is formed in the
atmosphere in the presence of sunlight through complex chemical reactions between precursor
emissions of VOC and NOx in the presence of sunlight. These reactions are accelerated by
sunlight and increased temperatures and, therefore, peak ozone levels typically occur during the
warmer times of the year.
2.2 NITROGEN OXIDES (NOX)
Nitrogen oxides include numerous compounds comprised of nitrogen and oxygen. Nitric oxide
(NO) and nitrogen dioxide (NO2) are the most significant nitrogenous compounds, in terms of
quantity released to the atmosphere. Generally, sources of NOx emissions may be categorized
either as stationary or mobile, and as combustion processes or noncombustion processes. Nitric
oxide is the primary nitrogen compound formed in high temperature combustion processes when
nitrogen present in the fuel and/or combustion air combines with oxygen. On a national basis,
total emissions of NOx from noncombustion stationary sources (such as chemical processes) are
small relative to those from stationary combustion sources (such as utility boilers).
2.2.1 HOW ARE NITROGEN OXIDES FORMED IN STATIONARY COMBUSTION SOURCES?
The formation of NOx from a specific combustion device is determined by the interaction of
chemical and physical processes occurring within the device. The three principal types of NOx
formations are:
EIIP Volume II
Thermal NOx: Formed through high temperature oxidation of the nitrogen found
in the high-temperature post-flame region of the combustion system. During
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combustion, oxygen radicals (O) are formed and attack atmospheric nitrogen
molecules to start the reactions that comprise the thermal NOx formation
mechanism:
O + N2 NO + N
N + O2
NO + O
N + OH
NO + H
Four factors influence thermal NOx formation:
-
Temperature;
-
Oxygen concentration;
-
Nitrogen concentration; and
-
Residence time.
Of these, temperature is the most important. Significant levels of NOx are usually
formed above 2200°F under oxidizing conditions, with exponential increases as
the temperature increases. Maximum thermal NOx production occurs at a slightly
lean fuel-to-air ratio due to the excess availability of oxygen for reaction within
the hot flame zone. Thermal NOx is typically controlled by reducing the peak and
average flame temperatures. If the temperature or the concentration of oxygen or
nitrogen can be reduced quickly after combustion, thermal NOx formation can be
suppressed or “quenched.”
12.2-2
Fuel NOx: Formed by the oxidation of fuel-bound nitrogen to NOx during
combustion. Nitrogen found in fuels such as coal and residual oils is typically
bound to the fuel as part of the organic compounds in the fuel. The rate of fuel
NOx formation is strongly affected by the mixing rate of the fuel and air, and by
the oxygen concentration. Although fuel NOx levels increase with increasing fuel
nitrogen content, the increase is not proportional. In general, the control strategy
for reducing fuel NOx formation involves increasing the fuel-to-air ratio. The
fuel-bound nitrogen is released in a reducing atmosphere as molecular nitrogen
(N2) rather then being oxidized to NOx. As with thermal NOx, controlling excess
oxygen is an important part of controlling fuel NOx formation.
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Prompt NOx: Formed in the combustion system through the reaction of
hydrocarbon fragments and atmospheric nitrogen. The name reflects the fact that
prompt NOx is formed very early in the combustion process. The formation of
prompt NOx is weakly dependent on temperature and is significant only in very
fuel-rich flames. It is not possible to quench prompt NOx formation, as it is for
thermal NOx formation.
The traditional parameters leading to complete fuel combustion (high temperatures, long
residence time, and high turbulence or mixing) all tend to increase the rate of NOx formation.
2.2.2 HOW ARE NITROGEN OXIDES FORMED IN STATIONARY NONCOMBUSTION
SOURCES?
Stationary noncombustion sources include various chemical processes, such as nitric acid and
explosives manufacturing. In these processes, the formation of NOx generally results from
nitrogen compounds used or produced in chemical reactions.
2.2.3 WHAT CHARACTERISTICS OF NITROGEN OXIDES DETERMINE THE TYPE OF AIR
POLLUTION CONTROL DEVICE USED FOR EMISSIONS CONTROL?
Characteristics of nitrogen oxides that impact the effectiveness of specific air pollution control
devices include:
NOx can be chemically reduced by reburning using natural gas. NOx can also be
reduced by injecting ammonia or urea at the proper temperature with or without a
catalyst.
The quantity of NOx formed during combustion depends on: the quantity of
nitrogen and oxygen available; temperature; level of mixing; and the time for
reaction. Management of these parameters can form the basis of control strategies
involving process control and burner design (low NOx burners and flue gas
recirculation).
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2.3 SULFUR DIOXIDE (SO2)
2.3.1 HOW IS SULFUR DIOXIDE FORMED?
Sulfur oxides, primarily SO2 and sulfur trioxide (SO3), are formed whenever any material that
contains sulfur is burned. From 95 to 100 percent of the total sulfur oxides emissions are in the
form of SO2, which is formed during combustion via the following reaction:
S + O2
SO
(12.2-1)
2
2.3.2 WHAT CHARACTERISTICS OF SULFUR DIOXIDE DETERMINE THE TYPE OF AIR
POLLUTION CONTROL DEVICE USED FOR EMISSIONS CONTROL?
Characteristics of SO2 that impact the effectiveness of specific air pollution control devices
include:
12.2-4
Sulfur can sometimes be removed from fuel prior to combustion. This may be a
cost effective way to reduce SO2 formation.
SO2 is chemically reactive. Therefore, control techniques that reduce pollutant
levels via chemical reaction (such as wet acid gas scrubbers and spray dryer
absorbers) are appropriate. Also, it can be removed by fluidized limestone bed
combustion.
Formation of SO2 occurs early in the primary flame and will occur even in
fuel-rich flames. As a result, combustion control techniques are not applicable to
reduce SO2 emissions.
Formation of SO3 is found to occur only in fuel-rich mixtures and can be
influenced by control of combustion conditions.
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2.4 VOLATILE ORGANIC COMPOUNDS (VOC)
2.4.1 HOW ARE VOLATILE ORGANIC COMPOUNDS FORMED?
The class of air pollutants referred to as volatile organic compounds includes hundreds of
individual compounds, each with its own chemical and physical properties. VOC are emitted
from combustion processes, industrial operations, solvent evaporation, and a wide variety of
other sources.
2.4.2 WHAT CHARACTERISTICS OF VOLATILE ORGANIC COMPOUNDS DETERMINE THE
TYPE OF AIR POLLUTION CONTROL DEVICE USED FOR EMISSIONS CONTROL?
Characteristics of VOC that impact the effectiveness of specific air pollution control devices
include:
Most VOC are adsorbable and may be collected by concentration on the surface of
a liquid or solid.
VOC are combustible and may be oxidized by thermal or catalytic incineration.
2.5 PARTICULATE MATTER (PM)
2.5.1 HOW IS PARTICULATE MATTER FORMED?
Particulate matter can be formed as the result of three processes:
Materials handling or processing (e.g., crushing or grinding ores, loading bulk
materials, sanding of wood, abrasive cleaning [sandblasting]);
Combustion can emit particles of noncombustible ash or incompletely burned
materials; and
Gas conversion reactions or condensation in the atmosphere.
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2.5.2 WHAT CHARACTERISTICS OF PARTICULATE MATTER DETERMINE THE TYPE OF
AIR POLLUTION CONTROL DEVICE USED FOR EMISSIONS CONTROL?
Characteristics of PM that impact the effectiveness of specific air pollution control devices
include:
Particle size and size distribution;
Particle shape;
Particle density;
Stickiness;
Corrosivity;
Condensation temperature;
Reactivity; and
Toxicity.
You must also consider these characteristics of the flue gas stream:
12.2-6
Gas flow rate;
Particulate loading;
Pressure;
Temperature;
Viscosity;
Humidity;
Chemical composition; and
Flammability.
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2.6 CARBON MONOXIDE (CO)
2.6.1 HOW IS CARBON MONOXIDE FORMED?
During the combustion of any carbonaceous fuel, CO can be formed as the result of two
mechanisms:
Incomplete combustion. The burning of carbonaceous fuels is a complex
chemical process. Carbon monoxide formed as the first step in the combustion
process is then converted to carbon dioxide (CO2) by combustion with oxygen at
temperatures greater than 1160°F. When less than the stoichiometric amount of
oxygen is present, CO is the final product of the reaction.
High-temperature dissociation of CO2. The bond energy for CO2 is moderately
low. At high temperatures CO2 easily dissociates to form CO and oxygen (O2).
At elevated temperatures, an increase in oxygen concentration tends to decrease CO levels not
only by allowing for complete combustion, but because reaction rates increase with temperature,
increasing the chance for collision between CO and O2 molecules.
2.6.2 WHAT CHARACTERISTICS OF CARBON MONOXIDE DETERMINE THE TYPE OF AIR
POLLUTION CONTROL DEVICE USED FOR EMISSIONS CONTROL?
Characteristics of carbon monoxide that impact the effectiveness of specific air pollution control
devices include:
The quantity of CO formed during combustion depends on: quantity of oxygen
available; temperature; level of mixing; and the time for reaction. Management of
these parameters can form the basis of control strategies involving process control
and burner design.
CO is combustible and can be oxidized to CO2.
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12.2-8
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3
CONTROL TECHNIQUES FOR CRITERIA
POLLUTANTS
A variety of practices and equipment are applied either individually or in combination, to reduce
the emissions of criteria pollutants. In general, these techniques can be classified into:
Process modifications. These are changes made to the chemical, physical, or
thermal process. Process modifications include:
-
Substitution of raw materials. For example, a facility could change the
solvent used in a chemical process to reduce VOC emissions.
-
Substitution of fuels. For example, an electrical utility could switch to
coal with a lower sulfur content, or use a pre-processed or alternative fuel.
-
Modification of the combustion unit or changing the conditions within the
combustion unit. For example, the temperature profile in a boiler can be
controlled to limit the formation of nitrogen oxides by the application of
combustion unit modifications such as low NOx burners.
Post-process modifications. Also referred to as “end-of-pipe” or “tailpipe”
modifications, these techniques are applied downstream of the process, after the
flue gas has passed through the combustion or reaction unit. For example,
ammonia can be injected into the post-combustion flue gas stream to reduce NOx
emissions.
3.1 HOW ARE APPROPRIATE AIR POLLUTION CONTROL DEVICES
SELECTED?
Selection of the appropriate air pollution control device may be based on the following criteria:
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The physical and chemical characteristics of the pollutant. For example,
particulate (solid) matter pollutants are controlled by different techniques and
equipment than gaseous pollutants. Also, the particle shape and size, as well as
12.3-1
CHAPTER 12 - CONTROL DEVICES
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chemical reactivity, abrasiveness, and toxicity of PM pollutants must be
considered.
Gas stream characteristics such as volumetric flow rate, temperature, humidity,
density, viscosity, toxicity, or combustibility may limit the applicability of a
specific APCD in some facilities.
Control efficiency of the device. Federal, state, or local regulations may dictate
specific emission limits for pollutants based on control efficiency.
Requirements for handling and disposal of collected waste. For example, wet
scrubber installations have to consider treatment of wastewater and dry scrubbers
produce quantities of dry fine particulate that must be disposed of.
Siting characteristics such as available space; ambient conditions; availability of
utilities such as power and water; availability of ancillary system facilities such as
waste treatment and disposal.
Economic considerations:
-
Capital costs including equipment costs, installation costs, and engineering
fees;
-
Operating costs including fuel, treatment chemicals, utilities, and
maintenance; and
-
Expected equipment lifetime.
An air pollution control device or process charge selected to reduce emissions of one pollutant
can result in increased emissions of another pollutant. For example, increasing the air-to-fuel
ratio (i.e., increasing the amount of oxygen available during combustion) is an effective
mechanism to decrease CO emissions, but it dramatically increases NOx emissions. Care must
be taken to ensure that the entire emission control system provides adequate control of all
emissions. Selection of APCDs is normally made on flue gas stream-specific characteristics,
pollutant characteristics, and desired control efficiency.
12.3-2
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CHAPTER 12 - CONTROL DEVICES
3.2 CONTROL OF NITROGEN OXIDES EMISSIONS
3.2.1 WHAT PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED TO
CONTROL NITROGEN OXIDES EMISSIONS?
Process controls typically used to control NOx emissions include fuel switching and fuel
denitrification.
Fuel Switching
Conversion to a fuel with a lower nitrogen content or one that burns at a lower temperature may
result in a reduction of NOx emissions. Combustion of natural gas or distillate oils tends to result
in lower NOx emissions than is the case for coal or heavy fuel oils. While fuel switching may be
an attractive alternative from the standpoint of NOx emission reductions, technical constraints
and the availability and costs of alternative fuels are major considerations in determining the
viability of fuel switching. Moreover, fuel switching may result in greater emissions of other
criteria pollutants.
Fuel Denitrification
Fuel denitrification of coal or heavy oils could, in principle, be used to control fuel NOx
formation. Denitrification currently occurs as a side benefit of fuel pretreatment to remove other
pollutants, such as pretreatment of oil by desulfurization and chemical cleaning, or solvent
refining of coal for ash and sulfur removal. The low denitrification efficiency and high costs of
these processes do not make them attractive on the basis of NOx control alone, but they may
prove cost effective on the basis of total environmental impact.
3.2.2 WHAT COMBUSTION AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED
TO CONTROL NITROGEN OXIDES EMISSIONS?
NOx reduction mechanisms applied during the combustion process include controlling the rate of
the fuel-air mixing, reducing oxygen availability in the initial (primary) combustion zone, and
reducing peak flame temperatures. These include:
Low NOx burners (refer to Section 4.3);
Natural gas burner/reburn (refer to Section 4.4);
Water/stream injection (refer to Section 4.5);
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CHAPTER 12 - CONTROL DEVICES
Staged combustion (refer to Section 4.6);
Flue gas recirculation (refer to Section 4.7);
Low excess air (refer to Section 4.8); and
Staged overfire air (refer to Section 4.9).
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3.2.3 WHAT POST-PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED
TO CONTROL NITROGEN OXIDES EMISSIONS?
Post-process controls are techniques applied downstream of the combustion unit. In post-process
control, NOx is reduced to nitrogen and water through a series of reactions with a chemical agent
injected into the flue gas. These emission control techniques include:
Selective catalytic reduction (refer to Section 4.1); and
Selective noncatalytic reduction (refer to Section 4.2).
Nonselective catalytic reduction (refer to Section 4.10).
Table 12.3-1 presents control efficiencies for the various APCDs used to reduce nitrogen oxide
emissions. The table presents average, maximum, and minimum control efficiencies reported in
the references by each unique combination of emission source and control device. Refer to
Appendix D for a complete description of how the data were compiled and presented.
Appendix D also contains tables that present control efficiencies reported in the references for
control devices not evaluated in this document.
3.3 CONTROL OF SULFUR DIOXIDE EMISSIONS
3.3.1 WHAT PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED TO
CONTROL SULFUR DIOXIDE EMISSIONS?
Process controls typically used to control SO2 emissions include fuel switching, coal washing,
coal gasification and liquefaction, desulfurization of oil and natural gas, and fluidized bed
combustion.
12.3-4
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CHAPTER 12 - CONTROL DEVICES
Fuel Switching
Approximately two-thirds of the sulfur dioxide emitted in the United States are from coal-fired
power plants. Many coal-fired facilities attempt to reduce these emissions by switching to coal
with a lower sulfur content, such as subbituminous coal which generally contains less sulfur than
bituminous coal.
Coal Washing
Much of the sulfur in coal is in pyrite (FeS2) or in mineral sulfate form, much of which can be
removed by washing or other physical cleaning processes. However, disposal of the solid or
liquid wastes formed during these processes can be difficult and/or expensive.
Coal Gasification and Liquefaction
Organic sulfur, which is part of the molecular structure of the coal, cannot be removed by
washing or other physical cleaning processes. Chemical desulfurization of organic sulfur from
coal is extremely expensive. Coal gasification and liquefaction can remove much of the organic
sulfur, but results in a substantial loss of total available heating value.
Desulfurization of Oil and Natural Gas
The sulfur in crude oils and natural gas can be removed easily and economically and the
elemental sulfur recovered as a by-product can be sold as a raw material. The steps in the
desulfurization of oil or natural gas are:
R-S + H2
H2S + R (where R represents any organic group)
H2S + 3/2 O2
HO
2
+ SO2
(12.3-1)
(12.3-2)
2H2S + SO2
2H O
+ 3S
(12.3-3)
2H2S + SO2
2H O
+ 3S
(12.3-4)
2
2
Fluidized Bed Combustion
Also, combustion of crushed coal in a bed of a sorbent material (fluidized-bed combustion) can
reduce SO2 emissions. Sulfur dioxide in the coal reacts with limestone or dolomite in the bed to
form gypsum.
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3.3.2 WHAT POST-PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED
TO CONTROL SULFUR DIOXIDE EMISSIONS?
Dry and wet scrubbing are the most common technologies to desulfurize flue gas. Slurries of
sorbent and water react with SO2 in the flue gas. Refer to Sections 4.11 through 4.13.
Table 12.3-2 presents control efficiencies for the various APCDs used to reduce SO2 emissions.
This table presents average, maximum, and minimum control efficiencies reported in the
references by each unique combination of emission source and control device. Refer to
Appendix D for a complete description of how the data are compiled and presented. For those
wanting additional information, Appendix D also presents control efficiencies reported in the
references for control devices not evaluated in this document.
3.4 CONTROL OF VOLATILE ORGANIC COMPOUNDS
3.4.1 WHAT PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED TO
CONTROL VOLATILE ORGANIC COMPOUNDS EMISSIONS?
Typical strategies are:
Change of coating formulation, such as conversion to water-based paint;
Change from a VOC-based coating to a non-liquid coating such as powder coat;
and
Change to coating methods that increase transfer efficiency and reduce total
coatings used per application.
3.4.2 WHAT POST-PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED
TO CONTROL VOLATILE ORGANIC COMPOUNDS EMISSIONS?
Typical post-process control devices of VOC are:
12.3-6
Carbon adsorber (refer to Section 4.14);
Incinerator (refer to Sections 4.15 through 4.17);
Floating-roof storage tank (refer to Section 4.18);
Vapor capture device during tank filling; and
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CHAPTER 12 - CONTROL DEVICES
Fluid capture, recycle, and reuse.
Table 12.3-3 presents control efficiencies for the various APCDs used to reduce VOC emissions.
This table presents average, maximum, and minimum control efficiencies reported in the
references by each unique combination of emission source and control device. Refer to
Appendix D for a complete description of how the data are compiled and presented.
Appendix D also contains tables that present control efficiencies reported in the references for
control devices not evaluated.
Note: Many VOC emission sources are processes that are not enclosed or contained and the
VOC are emitted into the ambient work area. Before the emissions can be routed to a control
device, they must first be captured. There are many types of capture systems (a laboratory hood
is a good example) and they seldom capture 100% of the emissions. Although the capture
efficiency of a system does not always affect the control efficiency of a downstream control
device, it does affect the estimate of overall emissions reduction and, thus, the emissions
estimate. Therefore, inventory preparers should be aware that, for some processes, not all of the
VOCs generated are captured and controlled. Where a capture system is used, they should talk
to facility personnel to get an idea of the efficiency of the system. The questions provided in
Section 1.4 about control devices can be used as a guide for obtaining information about capture
systems.
3.5 CONTROL OF PARTICULATE MATTER
3.5.1 WHAT PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED TO
CONTROL PARTICULATE MATTER EMISSIONS?
Process controls typically used to control particulate matter emissions include fuel switching,
coal cleaning, and good combustion practices.
Fuel Switching
Fuel type has a great impact on particulate matter emissions. PM emissions can be reduced by
fuel substitution. Coal and fuel oil contain a variety of noncombustible minerals and mineral
oxides that are collectively referred to as ash. In terms of fuel composition, ash content of fuel is
the major factor in determining PM emissions: the higher the ash content, the higher the amount
of PM emitted from combustion. Fuel substitution can have a significant impact on PM
emissions. Reductions in emissions of PM10 and PM2.5 resulting from fuel substitution are shown
in Tables 12.3-4 and 12.3-5.
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In many cases, switching fuels will impact more than one type of pollutant. For example,
substituting natural gas for coal to reduce PM emissions can also effectively reduce sulfur
dioxide and nitrogen oxides emissions. However, switching to low sulfur coal to reduce sulfur
dioxide emissions can increase PM emissions.
Coal Cleaning
Physical cleaning of coal can be used to reduce mineral matter. This decreases PM emissions
and increases the energy content of the coal, but may not always be cost effective.
Good Combustion Practices
Incomplete combustion can result in increased particulate emissions due to unburned carbon
material released as particulate matter. Particulate emissions can be controlled by following
“good combustion practices” that include design and operational elements such as:
Control of the amount and distribution of excess air in the combustion zone;
Adequate turbulence in the combustion zone to ensure good mixing;
High temperature zone to ensure complete burning; and
Sufficient residence time (1 - 2 seconds) at the high temperature.
These good combustion practices also limit CO and dioxin/furan emissions, but can increase the
formation of NOx.
3.5.2 WHAT POST-PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED
TO CONTROL PARTICULATE MATTER EMISSIONS?
Four classes of control equipment are used to remove PM from gas streams:
12.3-8
Mechanical collectors such as cyclones (refer to Section 4.19);
Electrostatic precipitators (refer to Section 4.20);
Fabric filters, also referred to as baghouses (refer to Section 4.21); and
Wet PM scrubbers (refer to Section 4.22).
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CHAPTER 12 - CONTROL DEVICES
Table 12.3-6 presents CE for the various APCDs used to reduce PM emissions. This table
presents average, maximum, and minimum CE reported in the references by each unique
combination of emission source and control device.
3.6 CONTROL OF CARBON MONOXIDE
Process controls typically used to control CO emissions include fuel switching, good combustion
practices; and CO catalyst.
3.6.1 WHAT PROCESS AIR POLLUTION CONTROL DEVICES ARE TYPICALLY USED TO
CONTROL CARBON MONOXIDE EMISSIONS?
Fuel Switching
Fuel substitution can be used as a technique to reduce CO emissions because CO emissions from
coal-fired combustion are usually higher than those from the combustion of oil or natural gas.
Note, however that fuel substitution may result in higher emissions of other pollutants.
Good Combustion Practices
CO emissions can be controlled by following “good combustion practices” because CO
emissions from well-operated boilers are usually quite low. Good combustion practices include:
Control of the amount and distribution of excess air in the combustion zone;
Adequate turbulence in the combustion zone to ensure good mixing;
High temperature zone to ensure complete burning; and
Sufficient residence time (1 - 2 seconds) at the high temperature.
CO Catalysts
CO oxidation catalysts are typically used on gas turbines to control CO emissions, especially
turbines that use steam injection which can increase CO and unburned hydrocarbons in the
exhaust. CO catalysts are also being used to reduce gaseous organic compounds, including
organic HAPs emissions.
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3.6.2 WHAT POST-PROCESS COMBUSTION AIR POLLUTION CONTROL DEVICES ARE
TYPICALLY USED TO CONTROL CARBON MONOXIDE EMISSIONS?
Post-process techniques to reduce CO emissions are based on treatment of the exhaust gas to
oxidize CO to CO2. Air pollution control devices used are:
Thermal oxidizers (refer to Section 4.15);
•
Catalytic oxidizers (refer to Section 4.16); and
Flares (refer to Section 4.17).
The most critical operating parameter, in terms of limiting CO emissions, is the air-to-fuel ratio.
There must be sufficient levels of oxygen available to ensure complete combustion of CO to
CO2.
Some catalysts that reduce emissions of CO may decrease SO2 emissions and increase in NOx
emissions.
Table 12.3-7 presents control efficiencies for the various APCDs used to reduce CO emissions.
This table presents average, maximum, and minimum control efficiencies reported in the
references by each unique combination of emission source and control device.
12.3-10
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TABLE 12.3-1
CONTROL EFFICIENCIES (%) FOR NOX BY SOURCE CATEGORY AND CONTROL DEVICE TYPE
CE Range (%)
Process
Chemical Manufacturing
Fuel Combustion-Coal
Fuel Combustion-Coal
Fuel Combustion-Coal
Boiler
Boiler
Boiler
Fuel Combustion-Coal
Fuel Combustion-Coal
Boiler
Boiler
Fuel Combustion-Coal
Boiler
Fuel Combustion-Distillate
Oil
Fuel Combustion-Distillate
Oil
Fuel Combustion-Distillate
Oil
Fuel Combustion-Distillate
Oil
Fuel Combustion-Coal
Boiler
Low Excess Air
Low NOx Burners
Natural Gas
Burners/Reburn
Overfire Air
Selective Catalytic
Reduction
Selective Non-catalytic
Reduction
Flue Gas Recirculation
Boiler
Low Excess Air
Boiler
Overfire Air
Boiler
Selective Catalytic
Reduction
Low-NOx Burner with
Selective Non-catalytic
Reduction
* Average of widely varying values.
90
Minimum
Value
Maximum
Value
Reference
5
45
b
5
35
50
30
55
70
f
c
5
63
30
94
b
d
45*
55*
f
2
19
20
45
f
90
b
80
f
b
50
12.3-11
CHAPTER 12 - CONTROL DEVICES
Fuel Combustion-Coal
Operation
Control Device Type
AcrylonitrileSelective Non-catalytic
Incinerator Stacks Reduction
Boiler
Flue Gas Recirculation
Average CE
(%)
Reference
80
a
CONTROL EFFICIENCIES (%) FOR NOX BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Fuel Combustion-Coal
Operation
Boiler
Fuel Combustion-Municipal
Waste
Fuel Combustion-Municipal
Waste
Fuel Combustion-Natural Gas
Fuel Combustion-Natural Gas
Fuel Combustion-Natural Gas
Fuel Combustion-Natural Gas
Fuel Combustion-Natural Gas
Boiler
Boiler
Boiler
Boiler
Boiler
Boiler
Fuel Combustion-Natural Gas
Boiler
Fuel Combustion-Natural Gas
Gas Turbines
Fuel Combustion-Natural Gas
Fuel Combustion-Natural Gas
Gas Turbines
Reciprocating
Engines
Gas Turbines
Boiler
Fuel Combustion- Natural Gas
Fuel Combustion-Natural Boiler
Gas
Incinerator
69
60
Reference
Minimum
Value
85
Maximum
Value
95
Reference
f
40
60
f
80
a
30
65
a
49
0
40
13
80
68
31
85
73
90
b
b
e
b
e
35
80
b
60
96
g
60
80
94
90
g
e
50
40
80
50
g
f
b
b
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Fuel Combustion Coal
Control Device Type
Low -NOx Burner with
Overfire Air and Selective
Catalytic Reduction
Low -NOx Burner with
Overfire Air
Selective Catalytic
Reduction
Selective Non-catalytic
Reduction
Flue Gas Recirculation
Low Excess Air
Low NOx Burners
Overfire Air
Selective Catalytic
Reduction
Selective Non-catalytic
Reduction
Selective Catalytic
Reduction
Water or Steam Injection
Selective Non-catalytic
Reduction
Staged Combustion
Low-NOx Burner with
Overfire Air
Average CE
(%)
CHAPTER 12 - CONTROL DEVICES
12.3-12
TABLE 12.3-1
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TABLE 12.3-1
CONTROL EFFICIENCIES (%) FOR NOX BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Reference
b
5
31
b
Overfire Air
24
47
b
Selective Catalytic
Reduction
Selective Non-catalytic
Reduction
Flue Gas Recirculation
70
80
b
35
70
b
40
65
b
Selective Non-catalytic
Reduction
Selective Non-catalytic
Reduction
Selective Catalytic
Reduction
Selective Non-catalytic
Reduction
50
70
a
50
75
a
35
70
b
Operation
Boiler
Control Device Type
Flue Gas Recirculation
Fuel Combustion-Residual Oil
Boiler
Low Excess Air
Fuel Combustion-Residual Oil
Boiler
Fuel Combustion-Residual Oil
Boiler
Fuel Combustion-Residual Oil
Boiler
Fuel Combustion-Utility Oil or
Natural Gas
Fuel Combustion-Wood
Boiler
Boiler
Mineral Products Industry
Glass Flue
Petroleum Industry
Process
Heaters
Process
Heaters
Petroleum Industry
Average CE
(%)
21
90
Reference
b
Minimum
Value
2
b
12.3-13
CHAPTER 12 - CONTROL DEVICES
Maximum
Value
31
Process
Fuel Combustion-Residual Oil
CONTROL EFFICIENCIES (%) FOR NOX BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
a
Air & Waste Management Association. 1992. Air Pollution Engineering Manual. Anthony J. Buonicore and Wayne T. Davis, editors, Van
Nostrand Reinhold, New York, New York.
b
EPA. 1992b. Summary of NOx Control Techniques and their Availability and Extent of Application. U.S. Environmental Protection
Agency, EPA 450/3-9200094.
c
Pratapas, J. and J. Bluestein. 1994. Natural Gas Reburn: Cost Effective NOx Control. Power Engineering, May 1994.
d
EPA. 1997. Performance of Selective Catalytic Reduction on Coal-Fired Steam Generating Units. U.S. Environmental Protection Agency,
Acid Rain Division.
e
EPA. 1995. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources, Fifth Edition, AP-42.
Supplements A, B, C, D, and E. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle
Park, North Carolina.
f
EPA, 1994a. Alternative Control Techniques Document - NOx Emissions from Utility Boilers. U.S. Environmental Protection Agency,
EPA-453/R-94-023. http://www.epa.gov/ttn/catc/dir1/nox_act.txt
g
EPA, 1994b. Alternative Control Techniques Document - NOx Emissions from Stationary Gas Turbines. U.S. Environmental Protection
Agency, EPA-453/R-93-007. http://www.epa.gov/ttn/catc/dir1/nox_act.txt
CHAPTER 12 - CONTROL DEVICES
12.3-14
TABLE 12.3-1
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TABLE 12.3-2
CONTROL EFFICIENCIES (%) FOR SO2 BY SOURCE CATEGORY AND CONTROL DEVICE TYPE
a
CE Range (%)
Process
Chemical Manufacturing
Operation
Boiler
Chemical Manufacturing
Sulfuric Acid
Industry
Fuel Combustion-Coal
Boiler
Control Device
Type
Wet Acid Gas
Scrubber
Scrubber, General
Average CE
a
(%)
Reference
Minimum
Value
90
60
Maximum
Value
99
99
Reference
b
b
12.3-15
CHAPTER 12 - CONTROL DEVICES
Wet Acid Gas
80
99
c
Scrubber
Fuel Combustion-Coal
Boiler
Spray Dryer
70
90
c
Absorberd
Fuel Combustion-Lignite Boiler
Wet Acid Gas
90
c
Scrubber
Fuel Combustion-Lignite- Incinerator
Spray Dryer
50
c
95
c
Municipal Waste
Absorber
a
Reported control efficiencies are for sulfur oxides (SOx).
b
EPA. 1981. Control Techniques for Sulfur Oxide Emissions from Stationary Sources. Second Edition. U.S. Environmental Protection
Agency, EPA 452/3-81-004.
c
Air & Waste Management Association. 1992. Air Pollution Engineering Manual. Anthony J. Buonicore and Wayne T. Davis, editors,
Van Nostrand Reinhold, New York, New York.
d
Calcium hydroxide slurry, vaporizes in spray vessel.
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE
CE Range (%)
Process
Automobile
Manufacturing
Can Coating
Operation
Bake Oven
Exhaust
Exterior Coating
Can Coating
Exterior Coating
Can Coating
Can Coating
Interior Coating
Interior Coating
Can Coating
Interior Coating
Chemical Manufacturing
Absorber Vent
Chemical Manufacturing
Absorber Vent
Chemical Manufacturing
AcrylonitrileAbsorber Vent
AcrylonitrileAbsorber Vent
Reactor Vents
Residue Tower
Bottoms
SOCMI Reactor
Waste Gas
Column
Chemical Manufacturing
Chemical Manufacturing
Chemical Manufacturing
Chemical Manufacturing
Chemical Manufacturing
Control Device
Type
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
Carbon Adsorber
Catalytic
Incinerator
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Carbon Adsorber
Flares
Average CE
(%)
Reference
90
a
90
a
90
90
a
a
90
a
99.9
Minimum
Value
90
Maximum
Value
Reference
a
95
97
a
95
97
a
98
95
99
a
a
CHAPTER 12 - CONTROL DEVICES
12.3-16
TABLE 12.3-3
a
99.9
a
97
99.9
a
a
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TABLE 12.3-3
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Degreasing- In-line Cleaner
Degreasing- Open Top Vapor
Cleaner
Dry Cleaning
Operation
General
General
Food Industry
Spiral Ovens
Food Industry
General
Whiskey
Manufacturing/
Warehousing
Can Production
General
General
General
General
General
General
General
General
Average CE
(%)
Reference
Carbon Adsorber
Carbon Adsorber
Thermal
Incinerator
Catalytic
Incinerator
Carbon Adsorber
Thermal
Incinerator
Carbon Adsorber
Catalytic
Incinerator
Flares
Thermal
Incinerator
Minimum
Value
Maximum
Value
65
39
95
95
95
b
b
90
a
85
b
95
95
98
Reference
a
a
a
90
b
99
99
c
c
99
c
c
12.3-17
CHAPTER 12 - CONTROL DEVICES
Fabric Coating
Fabric Coating
Petroleum
Solvent
General
General
Control Device
Type
Carbon Adsorber
Carbon Adsorber
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
General
General
Operation
General
Graphic Arts
Natural Gas
Processing
Natural Gas
Processing
Drying Ovens
Graphic Arts
Drying Ovens
Graphic Arts
Flexography
Graphic Arts
Flexography
Graphic Arts
Graphic Arts
Gravure Printing
Gravure Printing
Graphic Arts
Graphic Arts
Graphic Arts
Heatset Web
Offset
Heatset Web
Offset
Printing Presses
Printing Presses
Groundwater Treatment
Air Strippers
General
Graphic Arts
Control Device
Type
Thermal
Incinerator
Flares
Floating Roof
Tank
Catalytic
Incinerator
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Carbon Adsorber
Average CE
(%)
99
99
95
Reference
Minimum
Value
98
Maximum
Value
Reference
d
96
99
b
60
99
b
90
98
a
90
98
a
95
98
a
95
95
99.8
a
a
90
98
a
95
99.8
a
75
95
b
75
95
a
CHAPTER 12 - CONTROL DEVICES
12.3-18
TABLE 12.3-3
a
a
b
7/14/00
EIIP Volume II
7/14/00
EIIP Volume II
TABLE 12.3-3
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Operation
Air Strippers
Liquid Storage Tanks
Storage Tanks
Lithography
Magnetic Tape Manufacturing
Printing
Presses
Printing
Presses
Drying Ovens
Magnetic Tape Manufacturing
Drying Ovens
Magnetic Tape Manufacturing
Drying Ovens
Metallurgical Industry
Metallurgical Industry
Open Arc
Furnaces
Smelters
Mineral Manufacturing
Kilns
Mineral Products Industry
Kilns
Municipal Solid Waste
Landfill
Lithography
Spray Dyer
Absorber
Spray Dryer
Absorber
Thermal
Incinerator
Flares
Average CE
(%)
Reference
90
b
90
b
95
a
98
a
98
a
98
e
90
a
95
e
98
a
Minimum
Value
90
Maximum
Value
98
Reference
a
96
99
e
95
99
e
12.3-19
CHAPTER 12 - CONTROL DEVICES
Process
Groundwater Treatment
Control
Device Type
Thermal
Incinerator
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
Carbon
Adsorber
Catalytic
Incinerator
Thermal
Incinerator
Flares
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Pharmaceutical Industry
Operation
Vent Streams
Plywood Manufacturing
General
Printing Lines
Flexography
Printing Lines
Letterpress
Printing Lines
Lithography
Printing Lines
Surface Coating
Rotogravure
Operations
Rotogravure
Operations
Blow Down
Tanks
Bake Oven
Surface Coating
Bake Oven
Surface Coating
Surface Coating
Coating Line
Curing Oven
Exhaust
Drying Ovens
Printing Lines
Rubber Manufacturing
Surface Coating
Control Device
Type
Thermal
Incinerator
Thermal
Incinerator
Thermal
Incinerator
Thermal
Incinerator
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Scrubber, General
Catalytic
Incinerator
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Carbon Adsorber
Average CE
(%)
98
Reference
a
Minimum
Value
90
60
a
95
a
95
a
75
a
65
a
90
a
96
a
96
a
80
90
a
b
95
b
Maximum
Value
Reference
b
CHAPTER 12 - CONTROL DEVICES
12.3-20
TABLE 12.3-3
7/14/00
EIIP Volume II
7/14/00
EIIP Volume II
TABLE 12.3-3
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Petroleum Industry
Petroleum Industry
Petroleum Industry
Petroleum Industry
Petroleum Industry
Petroleum Industry
Petroleum Industry
Petroleum Industry
Petroleum Industry
Average CE
(%)
98
85
Reference
b
a
Minimum
Value
95
Maximum
Value
99
95
90
b
98
b
98
Reference
a
a
98
99
a
63
81
b
68
88
b
95
99
a
a
12.3-21
CHAPTER 12 - CONTROL DEVICES
Petroleum Industry
Pharmaceutical Industry
Control Device
Operation
Type
Fixed Roof Tank Carbon Adsorber
Fixed Roof Tank Vent Recovery
System
Floating Roof
Vent Recovery
Tank
System
General
Catalytic
Incinerator
General
Thermal
Incinerator
Petroleum Tank Flares
Cleaning
Petroleum Tank Thermal
Cleaning
Incinerator
Petroleum Tank Flares
Transfer
Petroleum Tank Thermal
Transfer
Incinerator
Vent Streams
Flares
Vent Streams
Carbon Adsorber
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Surface Coating
Operation
Drying Ovens
Surface Coating
Surface Coating
Entire Process
Flatwood
Paneling
Processes
General
General
Surface Coating
Surface Coating
Surface Coating
Surface Coating
Magnet Wire
Production
Metal Coating
Metal Coil
Coating
Metal Coil
Coating
Metal Coil
Coating
Paper film
Surface Coating
Paper film/foil
Surface Coating
Surface Coating
Surface Coating
Surface Coating
Control Device
Type
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Carbon Adsorber
Thermal
Incinerator
Thermal
Incinerator
Carbon Adsorber
Catalytic
Incinerator
Thermal
Incinerator
Thermal
Incinerator
Thermal
Incinerator
Carbon Adsorber
Average CE
(%)
95
90
Reference
b
Minimum
Value
Maximum
Value
Reference
a
94
b
90
a
a
90
b
a
a
90
a
90
a
90
90
95
80
95
a
95
a
95
b
90
90
CHAPTER 12 - CONTROL DEVICES
12.3-22
TABLE 12.3-3
b
a
7/14/00
EIIP Volume II
7/14/00
EIIP Volume II
TABLE 12.3-3
CONTROL EFFICIENCIES (%) FOR VOC BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Surface Coating
Operation
Paper film/foil
Surface Coating
Polymeric
Coating
Polymeric
Coating
Polymeric
Coating
Spray Booth
Solvent
Recovery
Solvent
Recovery
General
Surface Coating
Surface Coating
Surface Coating
Waste Solvent Reclamation
Waste Solvent Reclamation
Waste Treatment and Land
Disposal
Wastewater Industry
Wastewater Industry
a
Control Device
Type
Thermal
Incinerator
Carbon Adsorber
Catalytic
Incinerator
Thermal
Incinerator
Carbon Adsorber
Carbon Adsorber
Floating Roof
Tank
Flares
Average CE
(%)
98
Reference
b
95
a
98
a
98
a
90
a
98
Minimum
Value
Maximum
Value
Reference
95
a
98
a
99
a
a
Treatment
Carbon Adsorber
System
Water Filtration Carbon Adsorber
90
b
EPA. 1992a. Control Techniques for Volatile Organic Compound Emissions from Stationary Sources. U.S. Environmental Protection
Agency, EPA 453/R-92-018.
c
EPA. 1991. Control Technologies for HAPs. U.S. Environmental Protection Agency.
d
EPA. 1998. Stationary Source Control Techniques Document for Fine Particulate Matter. U.S. Environmental Protection Agency, EPA,
452/R-97-001
e
EPA. 1995. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources, Fifth Edition, AP-42.
Supplements A, B, C, D, and E. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle
Park, North Carolina.
12.3-23
CHAPTER 12 - CONTROL DEVICES
90
99
e
Air & Waste Management Association. 1992. Air Pollution Engineering Manual. Anthony J. Buonicore and Wayne T. Davis, editors,
Van Nostrand Reinhold, New York, New York.
POTENTIAL PM10 EMISSION REDUCTIONS WITH FUEL SWITCHING (%)a
Replacement Fuel
Industrial
Utility
Subbituminous
Residual
Original Fuel
Residual Oilb Natural Gas Distillate Oilc Subbituminous
Natural Gas
Oilb
Bituminous Coal
21.4%
62.9%
98.2%
99.0%
21.4%
69.5%
99.3%
Subbituminous Coal
-52.8%
97.7%
98.8%
-61.2%
99.2%
–
–
95.1%
97.4%
--97.9%
Residual Oilb
a
Source: EPA. 1998. Stationary Source Control Techniques Document for Fine Particulate Matter. U.S. Environmental Protection
Agency. EPA 452/R-97-001
b
Residual Oil includes No. 4, 5, and 6 fuel oil.
c
Distillate Oil is No. 2 fuel oil.
CHAPTER 12 - CONTROL DEVICES
12.3-24
TABLE 12.3-4
TABLE 12.3-5
POTENTIAL PM2.5 EMISSION REDUCTIONS WITH FUEL SWITCHING (%)a
Replacement Fuel
Industrial
Utility
7/14/00
EIIP Volume II
Subbituminous
Distillate Subbituminou Residual
Original Fuel
Residual Oilb Natural Gas
Natural Gas
Oilc
s
Oilb
Bituminous Coal
21.4%
7.4%
93.1%
99.0%
21.4%
14.8%
97.5%
Subbituminous Coal
--91.2%
98.8%
--96.8%
–
–
92.5%
99.0%
--97.0%
Residual Oilb
a
Source: EPA. 1998. Stationary Source Control Techniques Document for Fine Particulate Matter. U.S. Environmental Protection
Agency. EPA 452/R-97-001.
b
Residual Oil includes No. 4, 5, and 6 fuel oil.
c
Distillate Oil is No. 2 fuel oil.
7/14/00
EIIP Volume II
TABLE 12.3-6
CONTROL EFFICIENCIES (%) FOR PM BY SOURCE CATEGORY AND CONTROL DEVICE TYPE
CE Range (%)
Operation
Boiler
Fuel Combustion- Bagasse
Fuel Combustion- Coal
Boiler
Boiler
Fuel Combustion- Coal
Fuel Combustion- Coal
Boiler
Boiler
Fuel Combustion- Coal
Fuel Combustion- Coal
(anthracite)
Fuel Combustion- Coal
(anthracite)
Fuel Combustion- Coal
(bituminous)
Fuel Combustion- Coal
(bituminous)
Fuel Combustion- Lignite
Boiler
Boiler
Boiler
Fuel Combustion- Lignite
Boiler
Boiler
Boiler
Boiler
Average CE
(%)
Reference
Minimum
Value
20
99
b
90
90
99
65
b
b
98.4
c
Maximum
Value
60
Reference
a
99.9
b
a
99
90
95
a
b
50
99
b
98.4
99.4
c
Electrostatic
Precipitator
Fabric Filter
96
99.4
c
98.3
99.9
c
Electrostatic
Precipitator
Mechanical
Collector
95
99.5
a
60
80
a
12.3-25
CHAPTER 12 - CONTROL DEVICES
Process
Fuel Combustion- Bagasse
Control Device
Type
Mechanical
Collector
Wet PM Scrubber
Electrostatic
Precipitator
Fabric Filter
Mechanical
Collector
Wet PM Scrubber
Electrostatic
Precipitator
Fabric Filter
CONTROL EFFICIENCIES (%) FOR PM BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Fuel Combustion- Wood
Operation
Boiler
Fuel Combustion- Wood
Fuel Combustion- Wood
Boiler
Boiler
Fuel Combustion- Wood
Sewage Sludge Incineration
Charcoal Industry
Boiler
Boiler
Briquetting
Operation
Briquetting
Operation
Charcoal
Production
Charcoal
Production
Condenser Unit
Charcoal Industry
Chemical Manufacturing
Chemical Manufacturing
Chemical Manufacturing
Control Device
Type
Electrostatic
Precipitator
Fabric Filter
Mechanical
Collector
Wet PM Scrubber
Wet PM Scrubber
Fabric Filter
Average CE
(%)
Reference
Minimum
Value
93
Maximum
Value
99.8
Reference
a, b
98
b
95.9
65
99.9
95
a
b
90
b
95
60
99
99
a
a
99
b
Mechanical
Collector
Fabric Filter
65
b
99
a
Mechanical
Collector
Mechanical
Collector
65
a
CHAPTER 12 - CONTROL DEVICES
12.3-26
TABLE 12.3-6
b
a
90
98
b
7/14/00
EIIP Volume II
7/14/00
EIIP Volume II
TABLE 12.3-6
CONTROL EFFICIENCIES (%) FOR PM BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Chemical Manufacturing
Chemical Manufacturing
Iron and Steel Production
Coal Industry
Ferroalloy Industry
Control Device
Operation
Type
Condenser Unit Scrubbers,
General
Condenser Unit Thermal
Incinerator with
Wet PM Scrubber
Delsulfurization Fabric Filter
Drying Ovens
Wet PM Scrubber
Fuel Combustion- Wood
Fuel Combustion- Wood Bark
General
General
General
General
General
General
General
General
General
Petroleum Industry
General
General
Reference
96
b
96.7
c
Fabric Filter
Fabric Filter
Thermal
Incinerator
Wet PM Scrubber
Wet PM Scrubber
Electrostatic
Precipitator
Fabric Filter
Mechanical
Collector
Wet PM Scrubber
Electrostatic
Precipitator
96.3
Minimum
Value
Reference
b
98
99.9
b
96.3
98.7
c
79
96
b
c
92.1
83.8
95
93.3
85.1
99.9
c
c
d
95
d
c
85
a
b
99
80
99
Maximum
Value
99
c
12.3-27
CHAPTER 12 - CONTROL DEVICES
Zinc Smelting
Chemical Manufacturing
Ferroalloy
Electric Arc
Furnace
Furnace
General
Average CE
(%)
CONTROL EFFICIENCIES (%) FOR PM BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Iron and Steel Production
Metallurgical Industry
Metallurgical Industry
Operation
Gray Iron
Cupolas
Iron Foundry
Lead Smelters
Metallurgical Industry
Metallurgical Industry
Lead Smelters
Lead Smelters
Copper Smelting
Coke Production
Petroleum Industry
Multiple Hearth
Roaster
Open Hearth
Furnace
Preheater
Process Heaters
Petroleum Industry
Process Heaters
Soap Industry
Production Line
Wood Products
Recover Furnace
Wood Products
Recover Furnace
Copper Smelting
Reverberatory
Smelter
Iron and Steel Production
Control Device
Type
Fabric Filter
Reference
99
c
99.2
c
Minimum
Value
93.4
Maximum
Value
93.9
98
95
99
99
a
a
95
80
99
90
a
a
89
92.9
85
b
b
85
b
90
97.2
Reference
c
a
90
99
a
85
99
b
c
7/14/00
EIIP Volume II
Fabric Filter
Electrostatic
Precipitator
Fabric Filter
Mechanical
Collector
Electrostatic
Precipitator
Electrostatic
Precipitator
Wet PM Scrubber
Mechanical
Collector
Electrostatic
Precipitator
Mechanical
Collector
Electrostatic
Precipitator
Wet PM Scrubber
with Electrostatic
Precipitator
Electrostatic
Precipitator
Average CE
(%)
CHAPTER 12 - CONTROL DEVICES
12.3-28
TABLE 12.3-6
7/14/00
EIIP Volume II
TABLE 12.3-6
CONTROL EFFICIENCIES (%) FOR PM BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
CE Range (%)
Process
Metallurgical Industry
Metallurgical Industry
Medical Waste Incineration
Iron and Steel Production
Copper Smelting
Food Industry
Mineral Products Industry
Phosphate Industry
Phosphate Industry
Phosphate Industry
Polystyrene Production
Agriculture Industry
Petroleum Industry
Average CE
(%)
95
99
69
Reference
a
Minimum
Value
Maximum
Value
Reference
20
80
a
90
99.9
94
a
c
98
90
99.9
99
b
b
96
80
99.9
99
99
b
b
b
99
b
b
b
99
90
a
a
12.3-29
CHAPTER 12 - CONTROL DEVICES
Soap Industry
Control Device
Operation
Type
Roasters
Cold Electrostatic
Precipitator
Roasters
Hot Electrostatic
Precipitator
Rotary Kiln
Fabric Filter
Sinter Furnace Electrostatic
Precipitator
Smelters
Fabric Filter
Smokehouses
Wet PM Scrubber
Thermal Dryer Wet PM Scrubber
Thermal Dryer Electrostatic
Precipitator
Thermal Dryer Wet PM Scrubber
Thermal Dryer Wet PM Scrubber
Thermal Dryer Mechanical
Collector with
Fabric Filter
Thermal Dryer Mechanical
Collector with
Fabric Filter
Transfer Systems Fabric Filter
Vent Streams
Mechanical
Collector
CONTROL EFFICIENCIES (%) FOR PM BY SOURCE CATEGORY AND CONTROL DEVICE TYPE (CONTINUED)
a
Air & Waste Management Association. 1992. Air Pollution Engineering Manual. Anthony J. Buonicore and Wayne T. Davis, editors,
Van Nostrand Reinhold, New York, New York.
b
EPA. 1995. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources, Fifth Edition, AP-42.
Supplements A, B, C, D, and E. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research
Triangle Park, North Carolina.
c
EPA. 1991. Control Technologies for HAPs. U.S. Environmental Protection Agency.
d
EPA. 1998. Stationary Source Control Techniques Document for Fine Particulate Matter. U.S. Environmental Protection Agency, EPA
452/R-97-001
CHAPTER 12 - CONTROL DEVICES
12.3-30
TABLE 12.3-6
7/14/00
EIIP Volume II
7/14/00
EIIP Volume II
TABLE 12.3-7
CONTROL EFFICIENCIES (%) FOR CO BY SOURCE CATEGORY AND CONTROL DEVICE TYPE
CE Range (%)
Process
Chemical Manufacturing
Control Device
Type
Thermal
Incinerator
Catalytic
Incinerator
Thermal
Incinerator
a
b
c
Reference
Minimum
Value
95
Maximum
Value
95
Comments
a
99
96
Reference
a
a
a
90
90
b
90
98
a
98
c
EPA. 1995. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources, Fifth Edition, AP-42.
Supplements A, B, C, D, and E. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle
Park, North Carolina.
EPA. 1979. Control Techniques for Carbon Monoxide Emissions. U.S. Environmental Protection Agency, EPA 452/3-79-006.
Air and Waste Management Association. 1992. Air Pollution Engineering Manual. Anthony J. Buonicore and Wayne T. Davis, editors,
Van Nostrand Reinhold, New York, New York.
12.3-31
CHAPTER 12 - CONTROL DEVICES
Operation
Catalytic Process
for Acrylonitrile
Chemical Manufacturing Catalytic Process
for Acrylonitrile
Chemical Manufacturing Catalytic Process
for Phthalic
Anhydride
Chemical Manufacturing Condenser Unit Thermal
Incinerator
Fuel Combustion- Natural Incinerator
Thermal
Gas
Incinerator
General
General
Catalytic
Incinerator
General
General
Thermal
Incinerator
Metallurgical Industry
Open Arc
Flare
Furnaces
Metallurgical Industry
Furnaces
Flare
Average CE
(%)
CHAPTER 12 - CONTROL DEVICES
7/14/00
This page is intentionally left blank
12.3-32
EIIP Volume II
4
DESCRIPTIONS OF AIR POLLUTION
CONTROL DEVICES
4.1 SELECTIVE CATALYTIC REDUCTION (SCR)
4.1.1 WHAT POLLUTANTS ARE CONTROLLED USING SELECTIVE CATALYTIC
REDUCTION?
NOx is controlled using SCR. SCR is the most developed and widely applied post-process NOx
control technique used today.
4.1.2 HOW DOES SELECTIVE CATALYTIC REDUCTION WORK?
A reducing agent, usually diluted with water, steam, or air, is injected through a grid system into
the flue gas stream upstream of a catalyst bed enclosed in a reactor. On the catalyst surface, the
reagent reacts with the NOx to form molecular nitrogen and water. The rate of reaction of the
reagent and NOx is increased by the presence of excess oxygen. The reduction reaction is
illustrated in Figure 12.4-1.
Note: SCR is “selective” in that the reagent reacts primarily with NOx, not with O2 or other major
components of the flue gas.
The performance of an SCR system is influenced by five factors:
Flue gas temperature;
Reagent-to-NOx ratio;
NOx concentration at the SCR inlet;
Space velocity (measure of the volumetric feed capacity of a continuous-flow
reactor per unit residence time); and
Condition of the catalyst.
EIIP Volume II
12.4-1
CHAPTER 12 - CONTROL DEVICES
7/14/00
FIGURE 12.4-1. REMOVAL OF NOX BY SCR
(BABCOCK & WILCOX, 1992)
The primary variable affecting NOx reduction is temperature. Below the optimal temperature
range, which depends on the type of catalyst used, the activity of the catalyst is greatly reduced,
allowing unreacted reagent to slip through. On the other hand, extreme temperatures can damage
the catalyst. Figure 12.4-2 illustrates a typical SCR system.
4.1.3 WHAT REDUCING AGENT IS USED IN SELECTIVE CATALYTIC REDUCTION?
With an appropriate catalyst, ammonia (NH3) or an ammonia derivative (i.e., urea), could be used
as the reducing gas; however, the most commonly used material is NH3. The reduction reactions
for the SCR process are:
12.4-2
4NO + 4NH3 + O2
4N
2
+ 6H2O
(12.4-1)
2NO2 + 4NH3 + O2
3N
+ 6H2O
(12.4-2)
2
EIIP Volume II
7/14/00
CHAPTER 12 - CONTROL DEVICES
FIGURE 12.4-2. SCHEMATIC FLOW DIAGRAM FOR THE SELECTIVE CATALYTIC
REDUCTION METHOD OF NOX CONTROL (AWMA, 1992)
4.1.4 WHAT CATALYSTS ARE USED IN SELECTIVE CATALYTIC REDUCTION?
Catalyst formulation is the key to SCR system performance. The catalyst must reduce NOx
emissions without producing other pollutants or compounds that could damage the equipment
downstream. The formulations of the catalytically active phases are proprietary, but generally
fall into 3 categories of composition:
Base metal catalysts which typically contain titanium and vanadium oxides and
may also contain molybdenum, tungsten, and other elements. Base metal catalysts
are used at temperatures between 450 and 800°F.
Zeolite catalysts (crystalline aluminosilicate compounds) are used at high
temperature operations, between 675 and 1100°F.
EIIP Volume II
12.4-3
CHAPTER 12 - CONTROL DEVICES
7/14/00
Precious metal catalysts which contain metals such as platinum and palladium.
These are used in clean, low temperature (between 350 and 550°F) operations.
Additional compounds may be present to give thermal or structural stability or to increase surface
area. Catalyst beds may be constructed in a honeycomb, plate, or bed configuration.
4.1.5 WHAT ISSUES ARE OF CONCERN WHEN USING SELECTIVE CATALYTIC
REDUCTION?
Catalyst deactivation and residual ammonia (ammonia slip) in the flue gas are two key
considerations in SCR systems. Catalyst activity decreases with operating time due to fouling.
“Ammonia slip” is the unreacted ammonia that remains in the flue gas stream downstream of the
SCR. Ammonia slip occurs when there is not enough NOx in the flue gas to react with the
injected ammonia. Ammonia slip is an indication that the ammonia injection rate should be
reduced. As flue gas temperatures decrease, this excess ammonia can react with sulfur
compounds from the fuel (especially SO3) to form ammonium salts such as ammonium sulfate
and ammonium bisulfate. Ammonium sulfate is a fine particulate and contributes to plume
opacity. An increase in plume opacity can cause a facility to be out of compliance with state
and/or federal opacity limits. Ammonium bisulfate is highly acidic and sticky and can result in
fouling and corrosion when deposited downstream. Ammonia uptake by flyash can make
disposal or reuse of the ash more of a challenge.
Ammonia slip is controlled by careful injection of the ammonia or urea into regions of the
combustion unit with appropriate conditions (temperature, residence time, concentration) for the
reduction reaction to occur. Distribution of the ammonia that matches flue gas strata is the
important factor in control of ammonia slip. The amount of ammonia slip is usually monitored
and used to determine the ammonia injection rate. Many units operate with an ammonia slip of
less than 1 parts per million (ppm). Units are usually guaranteed to operate at less than 5 ppm.
4.1.6 WHAT WASTES RESULT FROM USING SELECTIVE CATALYTIC REDUCTION?
Other than the spent catalyst, SCR produces no waste. Spent catalyst is typically reactivated for
use as a reducing agent or the components are recycled for other uses. When disposal is
necessary, spent catalyst can be disposed of in approved landfills because EPA has determined
that spent catalyst is not a hazardous waste (ICAC, 1997).
12.4-4
EIIP Volume II
7/14/00
CHAPTER 12 - CONTROL DEVICES
4.2 SELECTIVE NONCATALYTIC REDUCTION (SNCR)
4.2.1 WHAT POLLUTANTS ARE CONTROLLED USING SELECTIVE NONCATALYTIC
REDUCTION?
NOx is controlled using SNCR. This air pollution control technique is sometimes referred to as
ammonia injection, even though most systems currently use urea injection.
4.2.2 HOW DOES SELECTIVE NONCATALYTIC REDUCTION WORK?
A reducing agent is injected into the NOx-laden flue gas stream in a specific temperature zone in
the upper combustion unit. The SNCR process requires proper mixing of the gas and the reagent,
and the mixture must have adequate residence time for the reduction reactions to occur. High
temperatures (between 1400 to 2000°F) are required to provide activation energy sufficient to
eliminate the need for the use of catalysts. The NOx is reduced to molecular nitrogen and water.
Note: SNCR is “selective” in that the reagent reacts primarily with NOx, not with O2 or other
major components of the flue gas. Also, SNCR differs from SCR in that no catalyst is used in
the former.
Five factors influence the performance of urea- or ammonia-based SNCR systems:
Flue gas temperature;
Reagent-to-NOx ratio;
NOx concentration in the flue gas entering the combustion unit;
Residence time; and
Mixing.
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4.2.3 WHAT REDUCING AGENTS ARE USED IN SELECTIVE NONCATALYTIC REDUCTION?
Ammonia or urea, with urea used most often. Ammonia is usually injected into the gas stream in
the gaseous state; urea is injected in the aqueous state and therefore requires a longer residence
time to volatilize.
The chemical reaction for the ammonia-based process is:
4NO + 4NH3 + O2 4N2 + 6H2O
(12.4-3)
The chemical reaction for the urea-based process is:
2NO + (NH2)2CO + ½O2 2N2 + 2H2O + CO2
(12.4-4)
4.2.4 WHAT ISSUES ARE OF CONCERN WHEN USING SELECTIVE NONCATALYTIC
REDUCTION?
Excess urea degrades to nitrogen, carbon dioxide, and unreacted ammonia. Also, as with SCR,
“ammonia slip” can occur with SNCR. To minimize ammonia slip, the SNCR must be designed
to ensure good distribution and mixing of injected ammonia or urea within the proper
temperature zone. Many units operate with an ammonia slip of less than 1 ppm. Units are
usually guaranteed to operate at less than 5 ppm.
4.2.5 WHAT WASTES RESULT FROM USING SELECTIVE NONCATALYTIC REDUCTION?
No solid or liquid wastes are generated in the SNCR process, other than ammonia slip.
4.3 LOW NOX BURNERS (LNB)
4.3.1 WHAT POLLUTANTS ARE CONTROLLED USING LOW NOX BURNERS?
Low NOx burners are used to inhibit the formation of NOx.
4.3.2 HOW DO LOW NOX BURNERS WORK?
Low-NOx burners inhibit NOx formation by controlling the mixing of fuel and air. Different
burner manufacturers use different hardware to control the fuel-air mixing, but all designs
essentially automate two methods of NOx reduction: low excess air, described in Section 4.8,
and staged overfire air, described in Section 4.9.
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Low NOx burners reduce:
The oxygen level in the primary combustion zone to limit fuel NOx formation;
The flame temperature to limit thermal NOx formation; and/or
The residence time at peak temperature to limit thermal NOx formation.
The most common design approach is to control NOx formation by carrying out the combustion
in stages:
Staged air burners, or delayed combustion LNBs, are two-stage combustion
burners which are fired fuel-rich in the first stage. They are designed to reduce
flame turbulence, delay fuel/air mixing, and establish fuel-rich zones for initial
combustion. The reduced availability of oxygen in the primary combustion zone
inhibits fuel NOx formation. Radiation of heat from the primary combustion zone
results in reduced temperature. The longer, less intense flames resulting from the
staged combustion lower flame temperatures and reduce thermal NOx formation.
Staged fuel burners also use two-stage combustion, but mix a portion of the fuel
and all of the air in the primary combustion zone. The high level of excess air
greatly lowers the peak flame temperature achieved in the primary combustion
zone, reducing thermal NOx formation. The secondary fuel is injected at high
pressure into the combustion zone through a series of nozzles which are
positioned around the perimeter of the burner. Because of its high velocity, the
fuel gas entrains furnace gases and promotes rapid mixing with first stage
combustion products. The entrained gases stimulate flue gas recirculation. Heat
is transferred from the first stage combustion products prior to the second stage
combustion and, as a result, second stage combustion is achieved with lower
concentrations of oxygen and lower temperatures than would normally be
encountered.
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4.3.3 WHAT ISSUES ARE OF CONCERN WHEN USING LOW NOX BURNERS?
LNBs are applicable to tangential and wall-fired boilers of various sizes but are not applicable to
other boiler types such as cyclone furnaces or stokers. For example, in cyclone furnaces,
combustion occurs outside of the main furnace. As a result, low NOx burner modification of the
furnace is not suitable for this combustion system design.
More specifically, staged air burners lengthen the flame configuration. As a result, staged air
burners are applicable only to installations large enough to avoid impingement on the furnace
walls. Staged fuel burners are designed only for gas firing.
4.3.4 WHAT WASTES RESULT FROM USING LOW NOX BURNERS?
In some cases, LNBs with coal combustion increase the levels of carbon-in-ash. This can result
in the ash requiring treatment as a waste, rather than being a marketable product.
4.4 NATURAL GAS BURNER/REBURN
4.4.1
WHAT POLLUTANTS ARE CONTROLLED USING NATURAL GAS
BURNER/REBURN?
NOx is controlled using natural gas burner/reburn. Also, as a secondary benefit, since it replaces
10 to 20 percent of the heat input from the primary fuel, sulfur dioxide emissions may be
reduced, depending on the sulfur content of the primary fuel. When coal is the primary fuel,
carbon dioxide, particulate and air toxics emissions are reduced.
4.4.2 HOW DOES NATURAL GAS BURNER/REBURN WORK?
In a reburn configured boiler, reburn fuel (natural gas, oil, or pulverized coal) is injected into the
upper furnace region to convert the NOx formed in the primary fuel’s combustion zone to
molecular nitrogen and water. Figure 12.4-3 is a schematic diagram of a typical reburn system.
There are several natural gas burner/reburn boiler configurations. In general, the overall process
occurs within three zones of the boiler:
12.4-8
Combustion zone. The amount of fuel (coal, oil, or gas) input to the burners in
the primary combustion zone is reduced by 10 to 20 percent. To minimize NOx
formation and to provide appropriate conditions for reburning, the burners or
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FIGURE 12.4-3. SCHEMATIC DIAGRAM OF A TYPICAL REBURN SYSTEM
(AWMA, 1992)
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cyclones may be operated at the lowest excess air consistent with normal
commercial operation.
Gas reburning zone. Reburn fuel (between 10 and 20 percent of boiler heat
input) is injected above the primary combustion zone. This creates a fuel-rich
region where hydrocarbon radicals react with NOx to form molecular nitrogen.
Recirculated flue gases may be mixed in with the reburn fuel before it is injected
to promote better mixing within the boiler.
Burnout zone. A separate overfire air system redirects air from the primary
combustion zone to a location above the gas reburning reaction zone to ensure the
complete combustion of any unreacted fuel and combustible gases. Separate
overfire air systems also generally require new boiler penetrations and retrofitted
ducting.
Operational parameters that affect the performance of reburn include:
Reburn zone stoichiometry;
Residence time in the reburn zone;
Reburn fuel carrier gas; and
Temperature and O2 level in the burnout zone.
Decreasing the reburn zone stoichiometry can reduce NOx emissions. However, decreasing the
stoichiometry requires adding a larger portion of fuel to the reburn zone, which can adversely
affect upper furnace conditions by increasing the furnace exit gas temperature.
4.4.3 WHAT ISSUES ARE OF CONCERN WHEN USING NATURAL GAS BURNER/REBURN?
There must be sufficient space in the furnace above the primary burners to allow installation of
the necessary equipment.
4.4.4 WHAT WASTES RESULT FROM USING NATURAL GAS BURNER/REBURN?
None.
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4.5 WATER/STEAM INJECTION
4.5.1 WHAT POLLUTANTS ARE CONTROLLED USING WATER/STEAM INJECTION?
NOx from gas turbines are controlled using water/stream injection.
4.5.2 HOW DOES WATER /STEAM INJECTION WORK?
Water or steam is injected into the gas turbine, reducing the temperatures in the NOx-forming
regions. The water or steam can be injected into the fuel, the combustion air, or directly into the
combustion chamber.
4.5.3 WHAT ISSUES ARE OF CONCERN WHEN USING WATER /STEAM INJECTION?
Both hydrocarbon and carbon monoxide emissions are increased by large rates of water injection.
Water injection can increase the rate of equipment corrosion. Although water injection usually
results in a 2 to 3 percent decrease in efficiency, it may result in an increase in power output.
With combustion turbines for example, the power increase results because fuel flow is increased
to maintain turbine inlet temperature at manufacturers’ specifications.
4.5.4 WHAT WASTES RESULT FROM USING WATER/STREAM INJECTION?
None.
4.6 STAGED COMBUSTION
4.6.1 WHAT POLLUTANTS ARE CONTROLLED USING STAGED COMBUSTION?
NOx from gas turbines are controlled using staged combustion.
4.6.2 HOW DOES STAGED COMBUSTION WORK?
Most gas turbines were originally designed to operate with a stoichiometric mixture (an air-tofuel ratio of 1.0). Several types of staging methods are used in order to reduce NOx emissions
from gas turbines. These include:
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Lean combustion;
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Lean premixed combustion; and
Two-stage rich/lean combustion.
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Lean Combustion
Lean combustion involves increasing the air-to-fuel ratio so that the peak and average
temperature within the combustor will be less than that of the stoichiometric mixture. In lean
combustion, the additional excess air cools the flame, which reduces the peak flame temperature
and reduces the rate of thermal NOx formation.
Lean Premixed Combustion
In a conventional combustor, air and fuel mixing and combustion take place simultaneously in
the combustion zone. As a result, wide variations in air-to-fuel ratios exist, and the combustion
of localized fuel-rich pockets produces significant levels of NOx emissions. Lean premixed
combustors, also known as two-stage lean/lean combustors, involve premixing of fuel and air at
very lean air-to-fuel ratios prior to introduction into the combustion zone. Premixing results in a
homogeneous mixture, which minimizes localized fuel-rich zones, resulting in greatly reduced
NOx formation rates.
Two-Stage Rich/Lean Combustion
Two-stage rich/lean combustors, also known as rich/quench/lean (RQL) combustors, burn fuelrich in the primary zone and fuel-lean in the secondary zone. Incomplete combustion from the
fuel-rich mixture in the primary zone produces lower temperatures (as compared to a
stoichiometric mixture) and higher CO and hydrogen (H2). The CO and H2 replace some of the
O2 available for NOx generation and also act as reducing agents for any NOx formed in the
primary zone. Thus, fuel nitrogen is released with minimal conversion to NOx. The lower peak
flame temperatures due to partial combustion also reduce the formation of thermal NOx. Before
entering the secondary zone, the combustion products of the primary zone pass through a lowresidence-time quench zone where the combustion products are diluted by large amounts of air or
water. This rapid dilution extinguishes the flames, cools the combustion products, and at the
same time produces a lean mixture. The combustion of the lean mixture is then completed in the
secondary zone under fuel lean conditions. This step minimally contributes to the formation of
fuel NOx because most of the fuel nitrogen will have been converted to N2 prior to the lean
combustion phase. Thermal NOx is minimized during lean combustion due to the low flame
temperature.
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4.6.3 WHAT ISSUES ARE OF CONCERN WHEN USING STAGED COMBUSTION?
Lean Combustion
The performance of lean combustion is directly affected by the primary zone equivalence air-tofuel ratio. The closer the ratio is to 1.0, the greater the NOx emissions. However, if the ratio is
reduced too far, CO emissions increase. This emissions tradeoff effectively limits the amount of
NOx reduction that can be achieved by lean combustion alone.
Lean Premixed Combustion
The primary factor affecting the performance of lean premixed combustors is the air-to-fuel ratio.
To achieve low NOx emissions levels, the air-to-fuel ratio must be maintained in a narrow range
near the lean flammability limit of the mixture. Lean premixed combustors are designed to
maintain this air-to-fuel ratio at the rated load. At reduced load conditions, the fuel input
requirement decreases. To avoid combustion instability and excessive CO emissions that would
occur as the air-to-fuel ratio reaches the lean flammability limit, all manufacturers’ lean premixed
combustors switch to diffusion-type combustion mode at reduced load conditions, which results
in higher NOx emissions.
Another factor that affects the performance of lean premixed combustors is the type of fuel used.
Natural gas produces lower NOx levels than do oil fuels, because natural gas has a lower flame
temperature, and the ability to premix with air prior to delivery into the second combustion stage.
When using liquid fuels, currently available lean premixed combustors require water injection to
achieve appreciable NOx reductions.
Two-Stage Rich/Lean Combustion
NOx emissions from two-stage rich/lean combustors are affected primarily by the air-to-fuel ratio
in the primary combustion zone, and by the quench air flow rate. If the air-to-fuel ratio is not
selected carefully in the fuel-rich zone, both thermal and fuel NOx formation can be increase.
Further NOx emissions can increase with reduced quench air flow rates, which in turn, increases
the air-to-fuel ratio in the lean combustion stage.
4.6.4 WHAT WASTE RESULTS FROM USING STAGED COMBUSTION?
None.
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4.7 FLUE GAS RECIRCULATION (FGR)
4.7.1 WHAT POLLUTANTS ARE CONTROLLED USING FLUE GAS RECIRCULATION?
Flue gas recirculation (FGR) is applied to reduce NOx formation.
4.7.2 HOW DOES FLUE GAS RECIRCULATION WORK?
A portion of flue gas is recycled back to the primary combustion zone. This system reduces NOx
formation by two mechanisms:
Heating in the primary combustion zone of the inert combustion products
contained in the recycled flue gas lowers the peak flame temperature, thereby
reducing thermal NOx formation.
To a lesser extent, FGR reduces thermal NOx formation by lowering the oxygen
concentration in the primary flame zone.
The recycled flue gas may be pre-mixed with the combustion air or injected directly into the
flame zone. Direct injection allows more precise control of the amount and location of FGR.
Note: In order for FGR to reduce NOx formation, recycled flue gas must enter the flame zone.
4.7.3 WHAT ISSUES ARE OF CONCERN WHEN USING FLUE GAS RECIRCULATION?
The use of FGR has several limitations. The decrease in flame temperature alters the distribution
of heat and can lower fuel efficiency. Because FGR reduces only thermal NOx, the technique is
applied primarily to natural gas or distillate oil combustion.
Flue gas recirculation requires modifications to the ductwork of the combustion unit. Additional
power is required to operate recirculation fans, making the operating cost of flue gas recirculation
higher than some other combustion techniques.
4.7.4 WHAT WASTES RESULT FROM USING FLUE GAS RECIRCULATION?
None.
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4.8 LOW EXCESS AIR (LEA)
4.8.1 WHAT POLLUTANTS ARE CONTROLLED USING LOW EXCESS AIR?
Low excess air is applied to reduce NOx formation.
Excess air is the amount of air (oxygen) above the level stoichiometrically required for
100 percent combustion of the fuel. Because mixing of air and fuel is not complete at all times in
all regions of the combustor, some excess air is required to ensure complete combustion of the
fuel and to prevent CO and smoke formation or excess carbon-in-ash.
4.8.2 HOW DOES LOW EXCESS AIR WORK?
Low excess air works by reducing levels of excess air to the combustor, usually by adjustments
to air registers and/or fuel injection positions, or through control of overfire air dampers. The
lower oxygen concentration in the burner zone reduces conversion of the fuel nitrogen to NOx.
Also, under excess air conditions in the flame zone, a greater portion of fuel-bound nitrogen is
converted to N2 therefore reducing the formation of fuel NOx.
4.8.3 WHAT ISSUES ARE OF CONCERN WHEN USING LOW EXCESS AIR?
Issues that can be associated with low excess air systems include:
Too little excess air can result in increased emissions of carbon monoxide or
unburned carbon smoke; and
Too little excess air can reduce flame stability.
4.8.4 WHAT WASTES RESULT FROM USING LOW EXCESS AIR?
None.
4.9 STAGED OVERFIRE AIR
4.9.1 WHAT POLLUTANTS ARE CONTROLLED USING STAGED OVERFIRE AIR?
Staged overfire air is applied to reduce NOx formation.
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4.9.2 HOW DOES STAGED OVERFIRE AIR WORK?
Staged overfire air works by:
Partially delaying and extending the combustion process. This results in less
intense combustion and cooler flame temperatures, thereby suppressing thermal
NOx formation.
Lowering the concentration of air in the burner combustion zone where volatile
fuel nitrogen is evolved, thereby suppressing fuel NOx formation.
Staged combustion, or off-stoichiometric combustion, combusts the fuel in two or more steps.
A percentage of the total combustion air is diverted from the burners and injected through ports
above the top burner level. The total amount of combustion air fed to the furnace remains
unchanged. Initially, fuel is combusted in a primary, fuel-rich, combustion zone. Combustion is
completed at lower temperatures in a secondary, fuel-lean, combustion zone. The
sub-stoichiometric oxygen introduced with the primary combustion air into the high temperature,
fuel-rich zone reduces fuel and thermal NOx formation. Combustion in the secondary zone is
conducted at a lower temperature, reducing thermal NOx formation.
Staged overfire air combustion involves firing the burners more fuel-rich than normal while
admitting the remaining combustion air through overfire air ports or an idle top row of burners.
4.9.3 WHAT ISSUES ARE OF CONCERN WHEN USING STAGED OVERFIRE AIR?
Staged overfire air systems provide less available oxygen in the primary combustion zone. This
can result in:
Increased emissions of CO, organic compounds, and visible emissions;
Reduced flame stability, and changed furnace heat release rates and flue gas exit
temperatures;
Increased upper furnace ash deposits, referred to as “slagging”; and
Increased corrosion due to a reducing atmosphere in the lower furnace.
4.9.4 WHAT WASTES RESULT FROM USING STAGED OVERFIRE AIR?
None.
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4.10 NONSELECTIVE CATALYTIC REDUCTION (NSCR)
4.10.1 WHAT POLLUTANTS ARE CONTROLLED USING NONSELECTIVE CATALYTIC
REDUCTION?
Primarily NOx, but NSCR reduces CO and hydrocarbons (HC) as well.
4.10.2 HOW DOES NONSELECTIVE CATALYTIC REDUCTION WORK?
NSCR technique is essentially the same as the catalytic reduction systems that are used in
automobiles applications. NSCR is achieved by placing a catalyst in the exhaust stream of the
engine.
NSCR technique is also referred to as three-way catalyst because it simultaneously reduces NOx,
CO, and HC to water, CO2, and N2. This conversion occurs in two discrete and sequential steps:
Step 1:
Step 2:
2CO
2H2
HC
+
+
+
O2
O2
O2
2CO2
2H2O
CO2 +
H2O
NOx
NOx
NOx
+
+
+
CO
H2
HC
CO2
H2O
CO2
N2
N2
H2O
+
+
+
+
N2
In the first step, excess oxygen is removed from the exhaust gas. Because CO and HC react more
readily with O2, the O2 content of the exhaust is kept below approximately 0.5 percent. This will
ensure adequate NOx reduction in the second step. Therefore, NSCR is applicable only to
carbureted rich-burn engines.
Typically, natural gas is used as the NOx reducing agent in NSCR. Natural gas is injected into
the exhaust stream ahead of the catalyst reactor and acts as a reducing agent for NOx. Figure
12.4-4 is a schematic diagram of a typical NSCR system.
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FIGURE 12.4-4. SCHEMATIC OF A NONSELECTIVE CATALYTIC REDUCTION SYSTEM DESIGN WITH
A SINGLE CALALYTIC REACTOR
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4.10.3 WHAT ISSUES ARE OF CONCERN WHEN USING NONSELECTIVE CATALYTIC
REDUCTION?
The main issue of concern with NSCR is its limited applicability resulting from the narrow range
of exhaust O2 level required for consistent NOx reduction. NSCR can be installed on new
engines or retrofit to existing units. However, because of the air-to-fuel ratio necessary for the
operation of NSCR, this control technique can be used on carbureted rich-burn engines, but not
to fuel-injected units.
Other issues of concern when using NSCR include:
Control of air-to-fuel ratio: In order to reduce NOx emissions while minimizing
CO emissions from the catalyst, the exhaust O2 concentration must be maintained
at approximately 0.5 percent by volume. This O2 level is accomplished by
maintaining the air-to-fuel ratio in a narrow band.
Exhaust temperature: The operating temperature range for various NSCR catalysts
is from 375o to 825o C (700o to 1500o F). For NOx reductions of 90 percent r
greater, the temperature range narrows to approximately 425o to 650o C (800o to
1200o F). Although this temperature range is based on a compilation of available
catalyst formulations, individual catalysts will have narrower operating
temperature range, and maximum reduction efficiencies may not be achievable
over the entire spectrum of exhaust temperatures for an engine operating in a
variable load application. Moreover, abnormal operating conditions, such as
backfiring, can result in excessive temperatures that damage the highly porous
catalyst surface, permanently reducing the emission reduction capability of the
catalyst.
Masking or poisoning of the catalyst: Masking occurs when materials deposit on
the catalyst surface and cover the active areas. Poisoning occurs then materials
deposit on the catalyst surface and chemically react with active areas. Masking
and poisoning reduce the catalyst’s reduction capacity. Masking agents include
sulfur, calcium, fine silica particles, and hydrocarbons. Poisoning agents include
phosphorus, lead, and chlorides. Examples of masking and poisoning containing
fuels include landfill and digester gas fuels.
4.10.4 WHAT WASTES RESULT FROM USING NONSELECTIVE CATALYTIC REDUCTION?
None.
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4.11 WET ACID GAS SCRUBBERS
4.11.1 WHAT POLLUTANTS ARE CONTROLLED USING WET ACID GAS SCRUBBERS ?
Wet acid gas scrubbers are used to control SO2 emissions.
4.11.2 HOW DO WET ACID GAS SCRUBBERS WORK?
In most large systems, a sorbent material is milled and mixed into a slurry and pumped to an
absorber reaction tank. Flue gas is fed to the reactor and the SO2 in the gas is absorbed,
neutralized, and partially oxidized as the result of coming in contact with the sorbent material.
Wet acid gas scrubbers occur downstream of the particulate control devices to avoid erosion of
the desulfurization equipment and possible interference of particulate matter with the scrubbing
process. The slurry falls to a perforated plate tray where additional SO2 is absorbed into the froth
created by the interaction of the flue gas and the slurry on the tray. The slurry then drains back
into the reaction tank. A fraction of the slurry is continuously diverted to the disposal
(dewatering) system. Refer to Figure 12.4-5.
4.11.3 WHAT SORBENT MATERIAL IS USED IN WET ACID GAS SCRUBBERS?
Lime or limestone is used as the sorbent material. Both processes are nonregenerable; the
reagent is consumed by the process and must be continually replaced. Lime scrubbing and
limestone scrubbing are very similar in equipment and process flow, except that lime is a much
more reactive reagent than limestone. The major advantage of limestone scrubbing is that the
absorbent material is abundant and inexpensive. The disadvantages include scaling (hard
plugging), equipment plugging (soft plugging), and corrosion. The advantages of lime scrubbing
include better utilization of the reagent and more flexibility in operations. The major
disadvantage is the high cost of lime relative to limestone.
4.11.4 WHAT ISSUES ARE OF CONCERN WHEN USING WET ACID GAS SCRUBBERS?
Several parameters must be controlled in a wet scrubber to ensure continuous operation. The pH
of the slurry is one of these. Early scrubbers suffered from severe scale and plugging problems.
Scaling (hard plugging) resulted from precipitation of limestone in piping and on other surfaces
if the pH was too high. A low pH indicates a high concentration of calcium sulfite in the slurry
and can cause plugging (soft plugging) in pipes and other passages. The final reaction product,
calcium sulfate, can also produce hard plugging if it precipitates due to changes in pH.
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FIGURE 12.4-5. SCHEMATIC PROCESS FLOW DIAGRAM FOR A
LIMESTONE-BASED SO2 WET SCRUBBING SYSTEM
(COOPER AND ALLEY, 1994)
Fresh lime or limestone slurry is introduced into the system to control pH in the scrubber slurry.
Since the volume of slurry in the scrubber vessel must remain relatively constant, a bleed stream
of slurry must also be withdrawn from the scrubber.
Stainless steel, commonly 317L or similar quality, in wet acid gas scrubbers must be protected
from corrosion due to high concentrations of chloride salts in the slurry which is normally limited
to a fixed value. These salts are controlled by replacement of liquid of the slurry. The
combination of slurry necessary to control pH and concentration of limestone in the fresh slurry
are both varied to satisfy both of these limitations.
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These operational factors affect SO2 removal. Other operational parameters, such as the
recirculation rate of the slurry and the spray atomization characteristics in the scrubber will also
affect acid gas removal performance.
4.11.5WHAT WASTES RESULT FROM USING WET ACID GAS SCRUBBERS ?
Wet acid gas scrubbers generate large quantities of spent slurry. This waste can be disposed of
by:
Ponding the spent slurry without dewatering. This is the simplest method, but
requires a large ponding area and the management of the site is expensive.
A combination of dewatering, secondary dewatering, and landfilling. This is the
most common disposal strategy in the United States.
Sulfite sludge can be mixed with flyash and lime to yield a material suitable for
landfilling.
Gypsum can be concentrated to a cake and sold for use in wallboard or fertilizer
manufacture.
4.12 SPRAY DRYER ABSORBERS (SDA)
4.12.1 WHAT POLLUTANTS ARE CONTROLLED USING SPRAY DRYER ABSORBERS?
Spray dryer absorbers (SDA) are used primarily to control SO2. SDA have been applied to utility
boilers, smaller industrial applications, and for combined hydrogen chloride (HCl) and SO2
control at waste-to-energy units.
Spray dryer absorbers are also referred to as spray dryers, spray absorbers, dry scrubbers, and
semi-wet scrubbers.
4.12.2 HOW DO SPRAY DRYER ABSORBERS WORK?
Unlike a wet scrubber, an SDA is positioned before the particulate matter collector. In an SDA,
a highly atomized or aqueous lime slurry is sprayed into an absorption tower so that the slurry
droplets dry as they contact the hot flue gas. Sulfur dioxide is absorbed by the slurry, forming
CaSO3/CaSO4. The liquid-to-gas ratio is such that the water evaporates before the droplets reach
the bottom of the tower. The dry solids are carried out with the gas and collected in a fabric filter
with the fly ash. Refer to Figure 12.4-6.
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FIGURE 12.4-6. SPRAY DRYER ABSORBER SYSTEM SCHEMATIC
(BABCOCK & WILCOX, 1992)
4.12.3 WHAT SORBENT MATERIAL IS USED IN SPRAY DRYER ABSORBERS?
Slaked lime is usually used as the sorbent.
4.12.4 WHAT ISSUES ARE OF CONCERN WHEN USING SPRAY DRYER ABSORBERS?
The reagent slurry feed into the spray dryer generally is a mix of two feed systems and these
systems require close monitoring and maintenance to ensure proper operation of the SDA. The
first feed system is the rich lime slurry. The slurry is usually produced by slaking quick lime
(CaO) at the plant site to produce hydrated lime (Ca(OH)2) in a water based slurry. High quality
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water is necessary for the slaking operation. The second feed is “dilution” water, which can
contain plant wastewater, river water, and landfill leachate.
Both of these feeds are used to control operation of the SDA. Acid gas removal is achieved by
regulating the rich lime slurry feed rate. The longer the slurry droplets take to dry, while
maintaining a liquid surface, the greater the chemical reactivity of the lime and resulting removal
of acid gases. This is accomplished by regulating the dilution water feed rate to control the SDA
outlet gas to a fixed temperature as low as possible while maintaining the temperature high
enough to ensure the drying of solids to protect ductwork and downstream particulate control
devices.
Controls for both feeds are linked because the slurry contains water and will also affect the outlet
gas temperature. Most systems measure the acid gas concentration and temperature in the outlet
gas of the SDA, and the feed rates are regulated based on these measurements.
Control problems may occur if the system can not feed enough rich lime slurry into the unit.
This can result from improper design of the dual feed control system or if the rich lime slurry is
not sufficiently reactive to produce proper acid gas control alone.
Another concern is the response time of the feed system to varying acid gas concentrations in the
flue gas stream. Some facilities mix both feeds in a tank prior to SDA injection. Acid gas
concentrations will vary with time. This will occur quickly for some processes and will be more
pronounced in smaller units. Therefore, the mix tank must be designed to change its lime
concentration quickly enough to respond to changes in acid gas concentration.
4.12.5 WHAT WASTES RESULT FROM USING SPRAY DRYER ABSORBERS?
Spray dryer absorbers generate dry particulate matter that is collected in downstream air pollution
control devices.
4.13 DRY INJECTION
4.13.1 WHAT POLLUTANTS ARE CONTROLLED USING DRY INJECTION?
Acid gas pollutants including SO2 and HCl are controlled using dry injection.
4.13.2 HOW DOES DRY INJECTION WORK?
Dry injection, often referred to as dry sorbent injection (DSI), involves the addition of a dry
reagent to the gas stream to react with acid gases present. The reagent may be injected into the
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combustion zone or into the downstream duct. The reaction products are collected in a
particulate collection device. In some cases, a portion of the collected reaction products is
reinjected to increase acid gas removal and decrease reagent consumption.
4.13.3 WHAT SORBENT MATERIAL IS USED IN DRY INJECTION?
Hydrated lime [Ca(OH)2] or soda ash [Na2(CO3)] is usually used as the sorbent material.
4.13.4 WHAT ISSUES ARE OF CONCERN WHEN USING DRY INJECTION?
The main issue of concern is determining the proper reagent feed rate appropriate for the level of
acid gas in the flue gas stream and making prompt changes, when necessary, in the feed rate to
compensate for changes in the acid gas flow rate.
4.13.5 WHAT WASTES RESULT FROM USING DRY INJECTION?
Dry injection generates dry particulate matter that is collected in a downstream particulate
collection device, usually a fabric filter.
4.14 CARBON ADSORPTION
4.14.1 WHAT POLLUTANTS ARE CONTROLLED USING CARBON ADSORPTION?
Carbon adsorption is applied to control emissions of gaseous pollutants, primarily organic
compounds. Carbon adsorption is commonly used to control VOC emissions from dry cleaners,
degreasing operations, publication gravure printing plants, chemical processing industry,
petroleum industry, and landfills. Carbon systems have also been developed for the adsorption of
sulfur oxides.
In contrast to incineration techniques that destroy the organic compounds, carbon adsorption
provides a favorable control option when the organic compounds in the emission stream are
valuable because recovery of the organics may be possible.
4.14.2 HOW DOES CARBON ADSORPTION WORK?
Adsorption is the concentration of a substance on the surface of a liquid or solid. The adsorbed
substance does not penetrate within the crystal lattice of the solid or dissolve within it, but
remains entirely on the surface. Adsorption is not the same as absorption, in which the
substance passes through the surface to become distributed throughout the phase.
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Carbon adsorption air pollution control techniques are based on the principle that if the
intermolecular forces between the adsorbent and the pollutant are greater than those existing
between the molecules of the pollutant, the pollutant will condense on the surface of the
adsorbent. The adsorptive capacity of the carbon bed tends to increase with the concentration,
molecular weight, diffusity, and boiling point of the gas phase organics and decrease with
increased temperature of the flue gas.
To allow gas vent streams containing organic compounds to come into contact with the activated
carbon, the carbon granules are usually arranged in either a vertical or horizontal vessel. Small
units are manufactured with the carbon in place (canisters). Larger units are constructed so that
the carbon granules are loaded after installation; two configurations are common:
Fixed-bed systems are non-moving beds of activated carbon that are alternately
placed on-line and regenerated. When a continuous emission stream is being
treated, at least one bed is on line and one bed is on stand-by or being regenerated
at any given time. When the first bed approaches its capacity, the emission stream
is redirected to the second bed and the first bed is regenerated.
Fluidized-bed systems contain one or more beds of loose, beaded activated
carbon. The emission stream is directed upward through the bed and the organic
compounds are adsorbed onto the carbon. The flow of the emission stream stirs
the carbon beads, causing them to fluidize and flow through the adsorber. Fresh
carbon beads are continuously metered into the bed and organic compounds-laden
carbon is removed for regeneration.
Because the amount of organics that can be adsorbed per unit mass of activated carbon increases
as the temperature decreases, the flue gas is often passed through a cooler before entering the
adsorbent bed. The cooled gas stream travels through the adsorbent bed where the organic
compounds are removed and the remaining flue gas vented or returned to the source process.
When the capacity limit of the adsorbent is reached, the carbon granules can be removed and
replaced (canister systems), regenerated in place, or removed for regeneration. The saturated
carbon bed is regenerated by direct contact with low pressure steam.
Carbon adsorption is sensitive to emission stream conditions. The presence of liquid or solid
particles, high boiling organics, or polymerized substances may require pretreatment procedures
such as filtration.
4.14.3 WHAT SORBENT MATERIAL IS USED IN CARBON ADSORPTION?
Activated carbon is the preferred adsorbent material to remove organic compounds from gas
streams. It is produced by heating wood charcoal to between 350 and 1000°C in a vacuum, or in
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air, steam, or other gases. The activation process distills hydrocarbon impurities from the
charcoal and exposes a larger free surface for possible adsorption. Activated carbon has a high
affinity for:
Nonpolar compounds;
High-molecular-weight materials; and
Compounds with low volatility.
4.14.4 WHAT ISSUES ARE OF CONCERN WHEN USING CARBON ADSORPTION?
One issue with the carbon adsorption technique is that the capacity of the adsorbent bed for
adsorbing organic compounds progressively deteriorates with use.
4.14.5 WHAT WASTES RESULT FROM USING CARBON ADSORPTION?
Activated carbon beds are usually regenerated with steam. The steam is condensed and the
condensate, along with the recovered hydrocarbons, are sent to a wastewater treatment facility.
4.15 THERMAL OXIDATION
4.15.1 WHAT POLLUTANTS ARE CONTROLLED USING THERMAL OXIDATION?
Thermal oxidation is applied as a post-process technique to control emissions of gaseous
pollutants, primarily CO and VOC. Given a high enough temperature and a long enough
residence time, combustion can oxidize virtually all hydrocarbons to carbon dioxide and water.
Thermal oxidizers, also known as thermal incinerators, or afterburners, are used for low
concentrations of organic compounds. The concentration of the VOC in the flue gas or the
concentration of organics in the air must be kept substantially lower than the lower explosive
limit.
4.15.2 HOW DOES THERMAL OXIDATION WORK?
Flue gas, air, and fuel (typically natural gas) are continuously delivered to the reactor, where the
fuel and air are combusted in the firing unit. The energy released by combustion of the fuel heats
the flue gas which passes through the reactor where the organic pollutants are reacted (oxidized)
to harmless endproducts. The oxidation reactions require an elevated temperature (1200 -
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2000°F) and a residence time of 0.2 to 2.0 seconds. Figure 12.4-7 shows a typical thermal
oxidizer.
4.15.3 WHAT ISSUES ARE OF CONCERN WHEN USING THERMAL OXIDATION?
Issues that can be associated with thermal oxidation systems include:
Thermal oxidation is not well suited to gas streams with highly variable flow rates
because the reduced residence time, and poor mixing decrease the completeness of
the combustion during increased flow rates. This causes the combustion chamber
temperature to fall, decreasing the destruction efficiency.
Combustion of organic gases represents an explosion hazard.
Thermal oxidizers that are not operating efficiently can produce air pollutants.
The incomplete combustion of many organic compounds can result in the
formation of aldehydes and organic acids.
If the heat from the fuel burned is not recovered for process needs, or some useful
purpose, it amounts to wasted energy. This also results in extra releases of CO2, a
greenhouse gas.
4.15.4 WHAT WASTES RESULT FROM USING THERMAL OXIDATION?
None.
4.16 CATALYTIC OXIDATION
4.16.1 WHAT POLLUTANTS ARE CONTROLLED USING CATALYTIC OXIDATION?
Catalytic oxidation is applied primarily to control CO and gaseous organic compounds, including
organic HAPs. CO oxidation catalysts are typically used on gas turbines that use steam injection
which can increase CO and unburned hydrocarbons in the exhaust.
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FIGURE 12.4-7. SCHEMATIC OF A THERMAL OXIDIZER (AWMA,
1992)
4.16.2 HOW DOES CATALYTIC OXIDATION WORK?
Catalytic oxidation is very similar to thermal oxidation. In catalytic oxidation the gases pass over
a catalyst bed that promotes oxidation at a lower temperature (650 - 800°F) than required for
thermal oxidation.
Catalytic oxidation is not applied as widely as thermal oxidation because catalytic oxidation is
more sensitive to pollutant characteristics and process conditions than thermal oxidation.
Figure 12.4-8 shows a typical catalytic oxidizer.
4.16.3 WHAT CATALYST MATERIAL IS USED IN CATALYTIC OXIDATION?
Catalysts include:
C
Metals in the platinum family; and
C
Oxides of copper, chromium, vanadium, nickel, and cobalt.
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FIGURE 12.4-8. SCHEMATIC OF CATALYTIC OXIDIZER ( AWMA, 1992)
4.16.4 WHAT ISSUES ARE OF CONCERN WHEN USING CATALYTIC
OXIDATION?
Issues that can be associated with catalytic oxidation systems include:
C
Catalysts are subject to poisoning by many elements that are present in industrial
emissions, particularly halogens, sulfur compounds, zinc, arsenic, lead, mercury,
and particulates;
C
High temperatures can decrease catalyst activity;
C
Combustion of organic gases represents an explosion hazard; and
C
Catalytic oxidizers that are not operating efficiently can produce air pollutants. The
incomplete oxidation of many organic compounds can result in the formation of
aldehydes and organic acids that may create additional air pollution problems.
4.16.5 WHAT WASTES RESULT FROM USING CATALYTIC OXIDATION?
Spent catalyst should be considered as a potential hazardous pollutant in the solid waste stream.
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4.17 FLARES
4.17.1 WHAT POLLUTANTS ARE CONTROLLED USING FLARES?
Flares are used to control CO and most gaseous organic compounds. Flares are most commonly
used for disposal of large quantities of unwanted flammable gases and vapors resulting from
process upsets and emergencies. Flares are used when the concentration of organics in the air
equals or exceeds the lower explosive limit level or when the heating value of the emission
stream cannot be recovered economically because of uncertain or intermittent flows. Flares are
primarily used in the petroleum and petrochemical industries.
4.17.2 HOW DO FLARES WORK?
Vent gas containing organics is fed to and discharged from a stack. Mixing and combustion of
the vent gas, air, and fuel take place above the stack exit in the atmosphere. Complete
combustion must occur instantaneously because there is no residence chamber. Flare combustion
efficiency is related to flame temperature, residence time of gases in the combustion zone, the
amount of oxygen available for combustion, and degree of flue gas/oxygen mixing.
Figure 12.4-9 shows a typical flare.
Flare configurations can be classified as:
Smokeless flares introduce steam or air to ensure the efficient gas/air mixing and
turbulence necessary for complete combustion. Smokeless flaring is required for
the destruction of organic compounds heavier than methane. Steam-assisted
smokeless flares are most common.
Nonsmokeless flares are used to destroy organic vapor streams that burn readily
and do not produce smoke.
Fired, or endothermic, flares require additional energy to ensure complete
oxidation of the waste streams such as sulfur and ammonia.
4.17.3 WHAT ISSUES ARE OF CONCERN WHEN USING FLARES?
Issues that can be associated with flares include:
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Combustion of organic gases represents an explosion hazard.
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FIGURE 12.4-9. TYPICAL OPEN FLARE ( AWMA, 1992)
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Flares that are not operating efficiently can produce air pollutants.
When a flare is not operating properly, incomplete combustion can occur. The
incomplete combustion of many organic compounds can result in the formation of
aldehydes and organic acids that may create additional air pollution problems.
4.17.4 WHAT WASTES RESULT FROM USING FLARES?
None.
4.18 FLOATING ROOF SYSTEMS
4.18.1WHAT POLLUTANTS ARE CONTROLLED USING FLOATING ROOF TANK SYSTEMS?
Floating roof tank systems are used by petroleum producing and refining, petrochemical and
chemical manufacturing, bulk storage and transfer operations, and other industries consuming or
producing organic liquids to reduce the air emissions of VOC that occur as the result of
evaporation.
4.18.2 HOW DO FLOATING ROOF TANK SYSTEMS WORK?
Three designs are used to reduce evaporative loss of liquids and vapors during the storage of
organic liquids
External floating roof tanks (EFRT) have an open-topped cylindrical steel shell
with a roof that floats on the surface of the stored liquid. The roof rises and falls
with the liquid level in the tank. The floating roof consists of a deck, fittings, and
a rim seal. The rim seal system is attached to the deck perimeter and contacts the
tank wall. The seal system slides against the tank wall as the roof is raised and
lowered. The external floating roof design limits evaporative loss of the stored
liquid to losses from the rim seal system and deck fittings (standing storage loss)
and any exposed liquid on the tank walls (withdrawal loss).
Internal floating roof tanks (IFRT) have both a fixed permanent roof and a
floating roof inside. The function of the fixed roof is not to act as a vapor barrier,
but to block the wind. The deck in internal floating roof tanks rises and falls with
the liquid level and either floats directly on the liquid surface (contact deck) or
rests on pontoons several inches above the water (noncontact deck). Noncontact
decks are the most common type currently in use. Both contact and noncontact
decks incorporate rim seals and deck fittings to reduce evaporative loss of the
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stored liquid. These tanks are freely vented by circulation vents at the top of the
fixed roof. The vents minimize the possibility of organic vapors accumulating in
the tank vapor space to concentration levels that approach the explosive range.
Domed External Floating Roof Tanks are usually the result of the retrofit of an
EFRT. These tanks have the same type of deck, deck fittings, and rim seals used
in EFRT as well as a fixed roof at the top of the shell like IFRT. Like the IFRT,
these tanks are freely vented by circulation vents at the top of the fixed roof.
4.18.3 WHAT ISSUES ARE OF CONCERN WHEN USING FLOATING ROOF TANK
SYSTEMS?
Deterioration of seals; inspection and maintenance are important for continued good service.
4.18.4 WHAT WASTES RESULT FROM USING FLOATING ROOF TANK SYSTEMS?
None.
4.19 MECHANICAL COLLECTORS
4.19.1 WHAT POLLUTANTS ARE CONTROLLED USING MECHANICAL COLLECTORS?
Coarse and medium particulate matter are controlled using mechanical collectors.
4.19.2 HOW DO MECHANICAL COLLECTORS WORK?
The five major types of mechanical collectors are settling chambers, elutriators, momentum
separators, centrifugal collectors, and cyclones. These devices are discussed below.
Settling Chambers
The simplest mechanical collectors are settling chambers, which rely on gravitational settling as a
collection mechanism. Settling chambers prevent excessive abrasion and dust loading in primary
collection devices by removing large particles from the gas stream.
There are two primary types of settling chambers: the expansion chamber and the multiple-tray
chamber. In an expansion chamber, the velocity of the gas stream is significantly reduced as the
gas expands into a large chamber. The reduction in velocity allows larger particles to settle out
of the gas stream. A multiple-tray settling chamber is an expansion chamber with a number of
thin trays closely spaced within the chamber, which causes the gas stream to flow horizontally
between them. An expansion chamber must be very large to collect any small particles, but
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multiple-tray chambers have lower volume requirements for collection of small particles (greater
than, or equal to, 15 microns).
Elutriators
Elutriators rely on gravitational settling to collect particles. An elutriator is made up of one or
more vertical tubes or towers in series, where the gas stream passes upward through the tubes.
Larger particles whose terminal settling velocity is greater than the upward gas velocity are
collected at the bottom of the tube, while smaller particles are carried out of the top of the tube.
Size classification of the collected particles can be achieved by using a series of tubes with
increasing diameters.
Momentum Separators
Momentum separators utilize both gravity and inertia to separate particles from the gas stream.
Separation is accomplished by forcing the gas flow to sharply change direction within a gravity
settling chamber through the use of strategically placed baffles. Typically, the gas first flows
downward and then is forced by the baffles to suddenly flow upwards. Inertial momentum and
gravity act in the downward direction on the particles, which causes larger particles to cross the
flow lines of the gas and collect in the bottom of the chamber. Momentum separators are capable
of collecting particles as small as 10 microns at low efficiency (10-20 percent).
Centrifugal Collectors
Centrifugal collectors, sometimes referred to as mechanically aided separators, rely on inertia as
a separation mechanism. The gas stream is accelerated mechanically, which increases the
effectiveness of the inertia separation. As a result, centrifugal collectors can collect smaller
particles than momentum separators. A common type of certrifugal collectors is the modified
radial blade fan. In this device, the gas stream enters at the center of the fan, perpendicular to the
blade rotation. The blades propel the particles across the gas flow lines, where they are
concentrated on the inside wall of the casing. From there, the particles are diverted into a
collection hopper while the gas continues out of the separator.
Cyclones
Cyclones are essentially cylinders with inlet and outlet ducts for the air stream. A vortex is
created in the cylindrical section of the cyclone either by injecting the air stream tangentially or
by passing the gas through a series of vanes. As the particulate-laden gas is forced to change
direction in the vortex, the inertia of the particles forces them to continue in the original
direction, collide with the outer wall, and slide downward to the bottom of the device to be
collected in a hopper. The cleaned airstream passes upward and out of the cyclone. Particle
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separation is a function of gas throughput and the cyclone cylindrical diameter. Particle removal
efficiency increases with increased flue gas velocity and decreases with decreased cylinder
diameter. However, above an upper limit of flue gas velocity, increased turbulence can reduce
particle removal efficiency.
Simple cyclones consist of an inlet, cylindrical section, conical section, gas outlet tube, and a
dust outlet tube. Figure 12.4-10 is a schematic diagram of a typical cyclone. A multiple cyclone,
or multiclone, consists of a number of small-diameter cyclones operating in parallel. This design
takes advantage of the high efficiency of small diameter tubes and is capable of treating large gas
volumes.
4.19.3 WHAT ISSUES ARE OF CONCERN WHEN USING MECHANICAL COLLECTORS?
The issues are:
Plugging of the dust outlet tube can affect the performance of cyclones.
With cyclones, abrasion can lead to leaks or rough areas on the surface of the
cylinder that can cause local turbulence, reducing the effectiveness of the vortex in
removing particles.
The efficiency of multiple cyclones can be decreased by hopper recirculation
which occurs when uneven pressure drops across the system result in reversed
flow of the exhaust stream in some areas of the multiple cyclone.
The abrasive wear from large particles and clogging from particles which
accumulate on the fan blades can affect the efficiency of centrifugal collectors.
4.19.4 WHAT WASTES RESULT FROM USING MECHANICAL COLLECTORS?
Mechanical collectors collect dry particulate waste. To decrease the problems associated with
handling fine dust, the collected particulate matter can be wetted in a pug mill into a clay-like
consistency, or pelletized before it is recycled or landfilled.
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FIGURE 12.4-10. SCHEMATIC FLOW
DIAGRAM OF A STANDARD CYCLONE
(COOPER AND ALLEY, 1994)
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4.20 ELECTROSTATIC PRECIPITATORS (ESP)
4.20.1 WHAT POLLUTANTS ARE CONTROLLED USING ELECTROSTATIC
PRECIPITATORS?
Particulate matter emissions are controlled using ESPs.
4.20.2 HOW DO ELECTROSTATIC PRECIPITATORS WORK?
Electrostatic precipitators use an electrostatic field to charge particulate matter in the flue gas
stream. The charged particles then migrate to a grounded collection surface. The collected
particles are periodically dislodged from the collection surface by vibration or rapping.
An ESP is essentially a large box with a series of electrodes and grounded plates. Figure 12.4-11
is a schematic diagram of a typical ESP. ESPs use electrical forces to move the particles out of
the flowing gas stream and onto the collector plates. Voltage is applied to the electrodes causing
the gas between the negatively-charged electrodes to break down electrically, forming a
“corona.” The ions generated in the corona follow electric field lines from the electrodes to the
collecting plates; establishing charging zones through which the particles must pass. Particles
passing through the charging zone intercept some of the ions, which become attached. As the
particles pass each successive wire, they are driven closer and closer to the oppositely charged
collecting walls, but the turbulence of the gas tends to keep them uniformly mixed with the gas.
The collection process is a competition between electrical and dispersive forces. Eventually, the
particles approach close enough to the walls so that the turbulence drops to low levels and the
particles are collected. Refer to Figure 12.4-12.
Once the particles are collected on the plates, they must be removed from the plates without
reentraining them into the gas stream. This is usually accomplished by knocking them loose
from the plates, allowing the collected layer of particles to slide down into a hopper, from which
they are evacuated. Because particulate tends to agglomerate, the ash layer is removed in sheets.
There are several common configurations for electrostatic precipitators:
Plate-wire precipitators are the most common ESP configuration. In a plate-wire
ESP, dirty gas flows into a chamber consisting of a series of discharge wire
electrodes that are equally spaced along the center line between adjacent collection
plates. Charged particles are collected on the plates as dust. Plate-wire ESPs can
handle large volumes of gas and are used in coal-fired boilers, cement kilns, solid
waste incinerators, paper mill recovery boilers, petroleum refining catalytic
cracking units, sinter plants, basic oxygen furnaces, open hearth furnaces, electric
arc furnaces, coke oven batteries, and glass furnaces.
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FIGURE 12.4-11. CUTAWAY VIEW OF AN
ELECTROSTATIC PRECIPITATOR
(COOPER AND ALLEY, 1994)
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FIGURE 12.4-12. PARTICLE CHARGING AND COLLECTION WITHIN AN ESP
(BABCOCK & WILCOX, 1992)
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Rigid discharge electrode (RDE) units have electrodes suspended from highvoltage frames located in the area above the gas passages. RDEs are currently the
most popular configuration of ESPs. The discharge eletrodes are centered in the
gas passages. In a common form of this design, sharp-pointed needles attached to
a rigid structure are used as high-voltage electrodes instead of the electrodes
hanging between plates of a plate-wire ESP. RDE units are typically used in the
pulp and paper, ferrous and non-ferrous metals, petrochemical, cement, and wasteto-energy industries, as well as, electric power generating plants.
Wet precipitators are plate-wire, flat-plate, or tubular ESPs operated with water
flow applied intermittently or continuously to wash the collected particles into a
sump for disposal. This configuration has the advantage that it eliminates
problems with re-entrainment. Disadvantages of the configuration include
increased complexity of the wash system and the fact that the collected slurry is
more difficult and more expensive to dispose of than dry particulate matter.
4.20.3
WHAT ISSUES ARE OF CONCERN WHEN USING ELECTROSTATIC
PRECIPITATORS?
The main issues affecting the control efficiency of an ESP are the design of the device and proper
maintenance. The design of an ESP for a particular application is based on characteristics of the
particulate matter that affect its ability to be collected and the gas volume flow rate. The ability
of the particulate matter to be collected is affected by the particle migration velocity. The
particle migration velocity is the rate at which the particle moves along the electric field lines
toward the walls, where they are collected. Particle migration velocity is based on the estimated
particle charge, mass of particles in the gas stream, and particle diameter and shape (size). These
estimations aren’t always exactly correct because the particulate actually consists of particles of a
wide range of sizes. Collection efficiency decreases as the particle diameter becomes smaller
down to about 0.5 microns when Brownian Motion effects cause movement toward the collection
surfaces. Therefore, the collection efficiency of PM10 and PM2.5 is much lower than for total PM.
The particle migration velocity is used to determine the specific collecting area (SCA) required to
achieve the desired collection efficiency. The SCA is the ratio of the collecting surface area to
the gas volume flow rate and is usually expressed in units of square feet of collection area per
thousand actual cubic feet per minute of gas flow (ft2/kacfm). The design total collecting area
(size of the ESP) is determined by multiplying the SCA by the gas volume flow rate. ESPs are
usually designed with more theoretical total collecting area than is required to achieve a
guaranteed control efficiency. This minimizes the possibility of not meeting the guarantee
because of changes in PM or flue gas characteristics. Thus, if flue gas parameters and particulate
matter characteristics are not considered when designing the ESP, the control efficiency will not
be at the desired level.
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The electrical fields must be properly maintained in order for the ESP to achieve the desired
control efficiency. Each electrical field in an ESP is composed of bus sections. If electrical
power is lost to a bus due to grounding or other reasons, the bus will be out of service. Bus
sections out of service directly in line between fields will reduce the control efficiency because
some of the particles miss multiple active fields. To account for this, the number of bus sections
per field in industrial ESPs has increased over the last couple of decades.
Simply operating the ESP will reduce the control efficiency over time. Non-removable dust
buildup on discharge and collecting surfaces will inhibit current flow and particle charge
resulting in fewer particles collected. Warping of components will shorten the distance from
discharge to ground, and corrosion will create sharp edges that cause arcing. Both of these
conditions reduce the discharge voltage and charge buildup on the particles, reducing the
collection ability of the particles.
4.20.4 WHAT WASTES RESULT FROM USING ELECTROSTATIC PRECIPITATORS?
With the exception of wet precipitators, which generate liquid slurries, ESPs generate dry
particulate waste. To decrease the problems associated with handling fine dust, the collected
particulate matter can be wetted in a pug mill into a clay-like consistency, or pelletized before it
is recycled or landfilled.
4.21 FABRIC FILTERS (FF)
4.21.1 WHAT POLLUTANTS ARE CONTROLLED USING FABRIC FILTERS?
Fabric filters, also referred to as baghouses, are used to control emissions of particulate matter
and are capable of achieving the highest particulate removal efficiencies of all the particulate
control devices. They do not have a declining collection effectiveness for smaller particles
compared to other control devices. However, fabric filters are generally designed to reduce
overall PM emissions to below an expected concentration when the inlet concentrations are
within a specified range. The expected outlet concentration remains relatively “constant” even
though the inlet concentration varies within the specified range. See Section 1.5 of this
document for a discussion of the efficiency of fabric filters when used in series with other control
devices.
4.21.2 HOW DO FABRIC FILTERS WORK?
A fabric filter system consists of several filtering elements (“bags”), a bag cleaning system, and
dust hoppers contained in a main shell structure. Fabric filters remove dust from a gas stream by
passing the stream through a porous fabric. The fabric does some of the filtering, but plays a
more important role by acting as a support medium for the layer of dust that quickly accumulates
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on it. The dust layer (“cake”) is responsible for the highly efficient filtering of small particles,
but also increases the resistance to gas flow.
The major particle collection mechanisms of fabric filters are:
Inertial impaction occurs as the flue gas stream flows through the fabric. As the
gas stream approaches the fabric fibers, it accelerates and changes direction to pass
around the fiber. Inertia will maintain the forward motion of the particles, and
they will impact onto the surface of the fabric filter.
Collection by diffusion occurs as a result of both fluid motion and the Brownian
(random) motion of particles. Diffusional effects are most significant for particles
less than 1 micron in diameter.
Interception or sieving occurs when a particle comes within one particle radius
of an obstacle.
There are three common fabric filter configurations; refer to Figure 12.4-13.
Reverse air baghouses operate by directing the dirty flue gas inside the bags so
that dust is collected on the inside surface of the bag. The bags are periodically
cleaned by reversing the flow of air. This causes the dust cake to fall from the
bags to a hopper below. In some configurations, the bags are shaken during the
reversed air flow.
Shaker baghouses are similar to reverse air units in that cleaning occurs on the
inside surface of the bags. Unlike reverse air units, a mechanical motion is used to
shake the bags and dislodge the accumulated dustcake.
Pulse jet baghouses have an internal frame (cages) to allow collection of the dust
on the outside of the bags. The dust cake is periodically removed by a pulsed jet
of compressed air into the bag that causes a sudden bag expansion. Dust is
primarily removed by inertial forces when the bag reaches maximum expansion.
The vigorousness of the cleaning method and the fit of the bag against the cage
may limit bag life and increase dust migration through the fabric. Pulse jet
baghouses sometimes use pleated cartridges instead of bags.
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FIGURE 12.4-13. FABRIC FILTER TYPES
(BABCOCK & WILCOX, 1992)
12.4-44
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Removal of the dust from the fabric is a crucial factor in the performance of a fabric filter. If the
dust cake is not adequately removed, the pressure drop across the system will increase to an
excessive level. If too much cake is removed, excessive dust leakage will occur while the new
cake develops.
Fabric selection (considering both material and type of weave) is important. The fabric must be
matched properly with both the gas stream characteristics, and the type of particulate. The
commonly used fabrics have very different abilities with respect to operating temperatures and
chemical content of the gas stream. A bag life of 3 to 5 years is common. Refer to Table 12.4-1.
Successful operation of a fabric filter system depends on the proper selection of fabric and
cleaning method and on an adequate air-to-cloth ratio. The air-to-cloth ratio (A/C), is a critical
design feature of a fabric filter system. The A/C ratio is an important indicator of the amount of
air that can be filtered in a given time when considering the dust to be collected, cleaning
method, fabric type, and the characteristics of the gas stream to be filtered. The A/C ratio is a
measure of the amount of gas driven through each square foot of fabric in the baghouse and is
given in terms of the number of cubic feet of gas per minute flowing through 1 square foot of
cloth. The A/C ratio is more correctly referred to as the media face velocity because it is not the
actual velocity of the gas stream through the openings in the fabric, but the velocity of the gas
approaching the cloth. In general, as the A/C ratio increases, the efficiency of impaction
collection increases and diffusional collection efficiency decreases. However, as the A/C ratio
continues to increase, there is an increased pressure drop, increased particle penetration, blinding
of fabric, need for more frequent cleaning, and reduced bag life.
4.21.3 WHAT ISSUES ARE OF CONCERN WHEN USING FABRIC FILTERS?
While many different types of media are used in fabric filters dust collection is not usually an
issue if filter bags are in good condition. Emissions may still vary based on the media used.
Bags coated with a porous surface membrane such as Teflon® are extremely effective. Felt tends
to be more effective than woven fabric since it collects new particulate better, just after bag
cleaning, before the dust cake reestablishes itself.
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TABLE 12.4-1
TEMPERATURE AND CHEMICAL RESISTANCE
OF SOME COMMON INDUSTRIAL FABRICS USED IN FABRIC FILTERS
Fabric
a
Dynel
Cottona
Nylona
Polypropylenea
Dacrona
Nomex®a
Teflon®a
Fiberglass
P84 (polyimide)b
Ryton (polypropylene
sulfideb
Recommended
Maximum
Temperature
F
160
180
200
200
275
400
400
550
500
Acid
Good
Poor
Poor
Excellent
Good
Fair
Excellent
Good
Fair
Base
Good
Good
Good
Excellent
Fair
Good
Excellent
Good
Good
375
Good
Good
500
Good
Excellent
Expanded PTFEc
Chemical Resistance
a
Cooper, C.D., and F.C. Alley. 1994.
Manufacturer’s literature.
c
Loeffler, Dietrich, and Flatt. 1988.
b
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Fabric filters have several limitations:
The media or, in the case of cartridge type filters, glue attaching the cartridge to its
end flanges limits flue gas temperature to about 550oF. Many types of material of
different properties and cost are available within this range.
The unit cleaning mechanism must be able to remove the dust cake well enough so
that its resistance to gas flow does not cause the pressure differential to exceed the
intended value across the bags. Hygroscopic material or condensation of moisture
can cause a permanent caking or “blinding” of the media. Some dusts are
generally removed from the bag, but enough residual cake is left so that, after
cleaning, a permanent flow resistance is provided. This would require a lower gas
velocity, meaning more filter media, to maintain the desired pressure drop. High
pressure drop across cleaned bags causes a rapid cleaning rate, shortening the life
of the bags.
The filter media may be subject to chemical attack. Acids, alkalis, etc., may attack
the media.
Hot or burning embers may enter the unit and damage the media.
Combustible dusts can create a fire hazard. Fine dust can create a fire or explosion
hazard.
These problems are dealt with in selecting the composition and construction of the filter media.
Filter bag cost is also a major consideration in selection.
Dust cleaning causes the bags to weaken and fail over time. It is necessary to maintain a desired
pressure drop across the bags to protect the media, to minimize the number of times the bags are
cleaned, and possibly to provide a constant gas flow from the emission process. In some units, a
single compartment in the fabric filter is cleaned at a time, triggered when a fixed pressure drop
is reached. Units with pulse jet cleaning usually clean a small number of bag rows when
triggered.
Ensuring control of emissions from a fabric filter is based on inspection and maintenance of the
bags and other components. Holes in bags cause jets of dirty gas that rapidly destroy surrounding
bags by abrasion. Inspections should be frequent enough to limit this damage. Dust sensors at
the compartment outlet may sense this problem during operation or during bag cleaning. Dust
falls on top of the tubesheet (see Figure 12.4-13) when a bag leaks during operation. In a pulse
jet collector, this may be noticed as a sudden increase on an opacity meter beyond the fabric filter
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outlet when the cleaning air pulse suspends dust already on the tubesheet floor. The
compartment with filter damage can be determined in this case.
The indications that bags are leaking require a prompt inspection of the bags and replacement of
the damaged filters. A delay causes excess emissions and additional bags to fail.
4.21.4 WHAT WASTES RESULT FROM USING FABRIC FILTERS?
Fabric filters generate dry particulate waste. To decrease the problems associated with handling
fine dust, the collected particulate matter can be wetted in a pug mill into a clay-like consistency,
or pelletized before it is recycled or landfilled.
4.22 WET PM SCRUBBERS
4.22.1 WHAT POLLUTANTS ARE CONTROLLED USING WET PM SCRUBBERS?
Wet PM scrubbers control PM and acid gases, with some control of organics. Wet PM scrubbers
are applied as a post-process technique to:
Scrub particulates from incinerator exhausts;
Control particulate and gaseous emissions simultaneously;
Control acid gases;
Control sticky emissions that would otherwise plug filter-type collectors;
Recover soluble dusts and powders; and
Control metallic powders such as aluminum dust that tend to explode if handled
dry.
4.22.2 HOW DO WET PM SCRUBBERS WORK?
Wet PM scrubbers remove particles from gas by capturing the particles in liquid droplets (usually
water) and separating the droplets from the gas stream. Wet PM scrubbers are configured to
create a closely packed dispersion of fine droplets to act as targets for particle capture. The goal
is to cause the tiny pollutant particle to be lodged inside the collecting droplet and then to remove
the larger droplet from the gas stream. In general, the smaller the target droplet, the smaller the
size of particulate that can be captured and the more densely the droplets are packed, the greater
the probability of capture.
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Particles are captured by liquid droplets through three mechanisms; refer to Figure 12.4-14:
Impaction of the particle directly into a target droplet;
Interception of the particle by the target droplet as the particle comes near the
droplet; and
Diffusion of the particle through the gas surrounding the target droplet until the
particle is close enough to be captured.
There are several types of wet PM scrubber configurations, differing in the systems used to create
the droplet dispersion:
Venturi scrubbers are highly effective particulate control devices, but they
consume large amounts of energy, resulting in high operating costs. Venturi
scrubbers generate fine droplet dispersion by pneumatically atomizing the
scrubbing liquid in a high-velocity zone called the venturi throat. Target droplets
are dispersed by accelerating the gas stream to a high velocity and then using this
kinetic energy to shear the scrubbing liquid into fine droplets. The accelerating
force comes primarily from gas-stream kinetic energy, usually injected into the
system by a fan.
Mechanically aided scrubbers create droplet dispersion by a whirling
mechanical device, usually a fan wheel or disk. Liquid is injected into or onto the
disk and mechanical energy is added to break the liquid into fine droplets.
Mechanically aided scrubbers differ from venturi scrubbers in that mechanical
energy is applied to the system while venturi scrubbers apply only pneumatic
shearing. Mechanically aided devices use lower fan energy than other devices; but
on a total energy-input basis use more energy because the collection energy comes
from supplemental, driven energy.
Pump-aided scrubbers are eductor-type venturi scrubbers that use high-velocity
liquid spray to entrain the gas and pull it through the unit. Most of the energy
input comes from the pressurized liquid stream.
Wetted filter scrubbers force the liquid and gas through a medium with small
openings. A filtration-like process occurs, with the particulate temporarily
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FIGURE 12.4-14. SCHEMATIC OF HOW WET PM
SCRUBBERS REMOVE PARTICLES (AWMA, 1992)
12.4-50
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sticking to the filter. Wetted filter scrubbers are sometimes used in series and
usually used for low particulate loadings.
Tray or sieve scrubbers have no large gas-directing baffles, but are simply
perforated plates (or trays) with small orifices that accelerate the gas stream. The
trays are held in a tower, usually immediately downstream of a venturi. A water
level is maintained above the trays (there are usually 2 or more trays). The
particulate is injected into the liquid stream, using the energy of the gas. The
geometrical relationship of the tray thickness, hole diameter and spacing, as well
as the impinger details, results in a high-efficiency device for the removal of small
particulate of less than 2m in mean diameter. Refer to Figure 12.4-15.
Impingement tray scrubbers are tray scrubbers with target baffles.
A critical component of effective wet scrubbing for PM is efficient removal of the residual
droplets or mist. Common mist eliminator configurations include:
Cyclonic droplet removal which uses centrifugal force. Tangential velocity is
created through the use of vanes, rotating elements, or tangential gas inlet into a
cylindrical vessel. The cyclonic action throws the liquid against the vessel wall,
where it drains by gravity or is trapped.
Chevron droplet removal which is applicable for vertical or horizontal gas flows.
Flue is zig-zag shaped, with blades running parallel to the surface. The inertia of
the droplet tends to carry it straight ahead, so the droplets impact on the blade
surface, accumulate, and drain.
Mist pads, are used to coalesce fine liquid droplets until they enlarge enough to
fall, by gravity or capillary action, out of the pad. These are most often used
where little or no particulate is present.
4.22.3 WHAT ISSUES ARE OF CONCERN WHEN USING WET PM SCRUBBERS?
The issues are:
Droplet entrainment in the flue gas can increase the opacity of the plume;
Wet systems cause more corrosion problems than dry systems; and
Solids build-up at the wet-dry interface can be a problem.
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FIGURE 12.4-15. TRAY- OR SIEVE-TYPE
SCRUBBER (CATENARY GRID SCRUBBERTM)
(AWMA, 1992)
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4.22.4 WHAT WASTES RESULT FROM USING WET PM SCRUBBERS?
Wet PM scrubbers generate a waste slurry. This slurry can present a waste water treatment
problem. The chemical and physical routine of the particulate matter being collected determine
the ultimate disposal method of the slurry. If a scrubber is used to remove organic vapors, it is
important that they are released at the waste water treatment process.
4.23 WHEN ARE MULTIPLE CONTROL DEVICES USED?
Multiple control device types may be used in combination to control either a single pollutant or
multiple pollutants. For example, mechanical collectors are often used with fabric filters to
control PM emissions. The mechanical collector collects large particles and the fabric filter
collects smaller particles. Also, SCR is often used with fabric filters to control NOx and PM
emissions. The devices are arranged in series, or tandem, relative to the flue gas stream. The
specific types of devices used and the order in which they are arranged is dependent on the
process, gas stream, and pollutant characteristics. The overall control efficiency for multiple
devices is likely to be around the efficiency of the last device in the series (see Section 1.5 of this
document).
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EFFECTS OF AIR POLLUTION
CONTROL DEVICE MALFUNCTIONS ON
EMISSIONS
Excess emissions due to a malfunctioning control device can significantly increase the annual
emissions of a source, even if the malfunctions occur for only a small percentage of the operating
time. These emissions can be difficult to quantify, but if they are not accounted for, statewide
emission inventories can be understated. For example, the efficiency of an electrostatic
precipitator can be altered as a result of changes in process, including feedstock changes, which
result in flow variation, changes in particle resistivity or other modifications of pollutant
characteristics. The effects of such changes on efficiency are not always analyzed or considered
when estimating emissions and compiling inventories. This section provides:
A brief discussion of how excess emissions from control device malfunctions can
affect statewide emission inventories; and
Methods for calculating excess emissions due to a malfunctioning control device.
5.1 EXCESS EMISSIONS FROM AIR POLLUTION CONTROL DEVICE
MALFUNCTIONS
5.1.1 WHAT ARE SOME EXAMPLES OF EXCESS EMISSIONS?
The following examples are taken from actual malfunction reports or other reports provided to
various state agencies:
Example 1 -- VOC emissions from a loading station
A malfunction was reported for a truck loading rack for gasoline and diesel in
which the pump to the vapor recovery unit (a carbon adsorber) failed for
55 minutes. According to the malfunction report provided to the permitting
agency, approximately 199 pounds (lb) of excess VOC emissions were released
during this incident. This was the only reported incident for the quarter. While
199 lb of unexpected VOC may not appear significant, the potential accumulated
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annual emissions that may result from multiple events at multiple facilities in an
inventory area could be significant.
Example No. 2 -- SO2 emissions from a manufacturing process.
A sulfur recovery unit malfunctioned for 3 hours which resulted in an overload of
process gases through an oxidizer. The facility estimated that 1 ton of SO2 in
excess of permitted levels was released during this malfunction.
Example No. 3 -- VOC emissions from a manufacturing process
In one instance, a dirty flame arrestor on an incinerator reduced the oxidizer
chamber temperature for 1 hour and 4 minutes. The facility estimated that 983 lb
of VOC in excess of permit levels was released during this malfunction.
5.1.2
WHAT ARE SOME SPECIFIC CAUSES OF EXCESS EMISSIONS FROM CONTROL
DEVICE MALFUNCTIONS?
These are just a few examples reported in one state’s excess emissions database:
Event
“The cause of the excursion was due to a bad dust collector pulse valve.”
Duration
1 hour, 13 minutes
“The LVHC stream was being combusted in the No. 1 combination boiler. 2 hours, 2 minutes
The flame scanner on the boiler malfunctioned and the burner flame was
not detected by the scanner. The indication of loss of flame by the
scanner resulted in the removal of the LVHC.”
“The heat exchangers were plugged with pulp.”
8 days
“Faulty pump seal.”
3 hours, 10 minutes
“Power failure in the system.”
13 hours, 53 minutes
“A fuse blew on the control panel for scrubber number 1940sr050, which 20 minutes
controls emissions for calciners number 1940ca010 and 1985ca010. The
failure of the control panel caused a shutdown of the thermal oxidizers for
both calciners and the opening of the designated emergency vent.”
“Scrubber not functioning properly.”
1 hour, 30 minutes
“The air pressure was lost to the system and the valve failed in the open
position.”
2 hours, 15 minutes
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5.2 IMPACT OF EXCESS EMISSIONS
5.2.1 WHY ARE MALFUNCTIONING CONTROL DEVICES A CONCERN?
Equipment malfunctions result in increased emissions and can be a recurring problem at some
facilities because of old or poorly maintained equipment or because of the nature of the process.
Excess emissions due to a malfunctioning control device can significantly increase the annual
emissions of a source, even if the malfunctions occur for only a small percentage of the operating
time.
The seriousness of the excess emissions problem has resulted in regulatory actions in many states
which require facilities to report all incidents of excess emissions to the regulatory agency.
However, there is no guarantee that all incidents are reported. Some states focus their
enforcement actions on sources that operate below their target control level more than 5 percent
of the time. Examples presented in this section show malfunction times of even 1 or 2 percent
per year can have a significant impact on emissions.
5.2.2 WHY IS IT IMPORTANT TO TRACK THESE EMISSIONS?
A few hours per month of excess emissions can quickly add up to 5 percent, 25 percent, or even
more than 50 percent of the expected emissions for the entire year, if the emissions inventory is
calculated on the basis of specified control levels.
For example, consider a source that is expected to emit 10 tons tpy of PM calculated on the basis
of a 99 percent control level using an ESP and a 1,000 tpy uncontrolled emission rate. If this
source operates at 4,800 hours per year, and if the ESP lost partial field voltage for only 4 hours
per month (i.e., 1 percent of the total operating hours) resulting in the control efficiency dropping
from 99 percent to 75 percent during the malfunction, actual annual emissions would increase
from 10 tpy to 11.4 tpy. This equals an increase in emissions of 14 percent. If these conditions
were typical for the source category, then the emission inventory for the source category would
need to be increased by 14 percent to correct for emissions from the APCD malfunctions.
5.2.3
HOW DO EXCESS EMISSIONS FROM AIR POLLUTION CONTROL DEVICE
MALFUNCTIONS AFFECT EMISSION INVENTORIES?
Excluding excess emissions can result in an understated annual emission inventory. Emission
inventories are typically based on the “normal” level of emissions specified in rules that apply to
a set of sources. However, both federal and state rules may explicitly allow for short-term
exceedance of the normal control level or emission limits (whether due to malfunctions, startup
and shutdown, or other conditions). One example is the new source performance standard
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(NSPS) for electric utility steam generating units (40 CFR Section 60.40 Subpart Da;
44 FR 33613, June 11, 1997). This standard incorporates compliance provisions to allow for PM
and SO2 exceedances during startup, shutdown, or malfunction, given “emergency conditions”
are implemented to minimize emissions during these events.
The inventory preparer should be aware that “excess emissions or malfunction” reports are
required by most state agencies. When preparing the inventory, these reports should be reviewed
for applicable information to determine the duration and degree of malfunctions.
5.3 ACCOUNTING FOR EXCESS EMISSIONS IN AN EMISSION
INVENTORY
5.3.1 WHAT IS THE EFFICIENCY OF THE CONTROL DEVICE DURING PROCESS UPSET
CONDITIONS?
Data are not always available to offer quantitative estimates of control device efficiencies during
process upsets. Generally, only process upsets that overload the control device system will affect
the amount of emissions released. You should consult with process engineers and other
experienced emission inventory preparers to determine these effects on emissions. Most often,
state compliance and permitting staff will be the best sources of information regarding the
expected effects.
5.3.2 HOW CAN RELEASES DURING CONTROL DEVICE MALFUNCTIONS BE
CALCULATED?
If you know or can estimate control efficiencies during malfunction conditions, the emission
calculations are straightforward. Appendix E contains two example calculations where
malfunction conditions are known. Example E-1 shows the calculated emissions increase for a
coke and coal-fired boiler with an ESP that loses partial field voltage for several hours each
month. Example E-2 shows the emissions increase for a wood dryer that emits PM10 and VOC.
The tables and figures in Appendix F show the emission increases for other scenarios in which
you have an estimate of the emission rate increase during a malfunction.
Note, that some other types of malfunction estimates are more difficult, if not impossible, to
calculate accurately. Conditions such as the following will require greater use of engineering
judgment in developing estimates:
12.5-4
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VOC emission increases from enclosed systems (e.g., vapor recovery systems)
that have partial loss of vacuum or air flow;
Fugitive losses; and
Irregular emissions during process startup and shutdown, including particulate
emissions shortly after fabric filter shakeout.
5.3.3 FOR THOSE CASES IN WHICH EXCESS EMISSIONS FROM CONTROL DEVICE
MALFUNCTIONS CAN BE REASONABLY ESTIMATED, HOW CAN YOU COLLECT THE
RELEVANT DATA?
In some cases, these calculations may already be in your inventory. For example, utility boilers
will have likely incorporated malfunction emissions into the quarterly emission reports that they
must file (required by the NSPS) with your state agency. It is important to make sure that these
emission estimates are incorporated into the emission inventory.
If your state requires facilities to estimate emission exceedances during malfunctions, compliance
staff may have records of these estimates. However, this type of information for sources other
than utility boilers is rarely transferred from the compliance or permitting staff files to the state
emission inventory files. You will often have to confer with the compliance or permitting staff to
obtain these figures. Compliance or permitting staff are probably the best sources of information
regarding how frequently sources operate with malfunctioning control equipment, and regarding
the estimated magnitude of these emissions. You can supplement this information with other
data from regulations, engineering guidance, and other sources. Appendix G provides a general
list of data sources.
5.4 CONCLUSION AND COMMENT SOLICITATION
The information presented in Section 5 of this document has been derived from preliminary
conversations with permitting and inventory staff in only a few states. However, it is evident that
excess emissions due to control device malfunctions may have a significant impact on individual
source emissions and on statewide emission inventories that are based strictly on emission levels
specified in regulations or permits. Furthermore, excess emissions from control device
malfunctions may have a more severe impact on source compliance than has been previously
expected or reported.
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The EIIP Point Sources Committee would like to improve our understanding of the actual impact
of control device malfunctions on emission inventories and would like to share this data with
emission inventory preparers throughout the United States. If you have comments on this
document, and particularly if you have data that can refine our understanding of this subject,
please contact us.
Roy Huntley, EIIP Point Sources Committee Co-Chair
Emission Factor and Inventory Group (MD-14)
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
E-Mail: [email protected]
Phone: (919) 541-1060
Fax: (919) 541-0684
Bob Betterton, EIIP Point Sources Committee Co-Chair
South Carolina Department of Health and Environmental Control
Bureau of Air Quality
2600 Bull Street
Columbia, SC 29201
E-Mail: [email protected]
Phone: (803) 898-4292
Fax: (803) 898-4117
12.5-6
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REFERENCES
Air & Waste Management Association. 1992. Air Pollution Engineering Manual. Anthony J.
Buonicore and Wayne T. Davis, editors, Van Nostrand Reinhold, New York, New York.
Babcock & Wilcox Company. 1992. Steam/its generation and use. 40th Edition. Steven C.
Stulz and John B. Kitto, editors, Babcock & Wilcox Company, Barberton, Ohio.
Cooper, C.D., and F.C. Alley. 1994. Air Pollution Control: A Design Approach. Waveland
Press, Prospect Heights, Illinois.
EPA. 1998. Stationary Source Control Techniques Document for Fine Particulate Matter. U.S.
Environmental Protection Agency, EPA 452/R-97-001.
EPA. 1997. Performance of Selective Catalytic Reduction on Coal-Fired Steam Generating
Units. U.S. Environmental Protection Agency, Acid Rain Division.
EPA. 1996a. Assessment of Performance Capabilities of LNBs (Low NOx Burners) Based on
reported Hourly CEM Data Through the Second Quarter of 1996. U.S. Environmental
Protection Agency, Acid Rain Division.
EPA. 1996b. Distributions of NOx Emission Control Cost-Effectiveness by Technology.
Environmental Protection Agency, Acid Rain Division.
U.S.
EPA, 1994a. Alternative Control Techniques Document - NOx Emissions from Utility Boilers.
U.S. Environmental Protection Agency, EPA-453/R-94-023.
http://www.epa.gov/ttn/catc/dir1/nox_act.txt.
EPA, 1994b. Alternative Control Techniques Document - NOx Emissions from Stationary Gas
Turbines. U.S. Protection Agency, EPA-453/R-93-007.
http://www.epa.gov/ttn/catc/dir1/nox_act.txt.
EPA. 1995. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and
Area Sources, Fifth Edition, AP-42. Supplements A, B, C, D, and E. U.S. Environmental
Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, North
Carolina.
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EPA. 1992a. Control Techniques for Volatile Organic Compound Emissions form Stationary
Sources. U.S. Environmental Protection Agency, EPA 453/R-92-018.
EPA. 1992b. Summary of NOx Control Techniques and their Availability and Extent of
Application. U.S. Environmental Protection Agency, EPA 450/3-9200094.
EPA. 1991. Control Technologies for HAPs. U.S. Environmental Protection Agency.
EPA. 1981. Control Techniques for Sulfur Oxide Emissions from Stationary Sources. Second
Edition. U.S. Environmental Protection Agency, EPA 452/3-81-004.
EPA. 1979. Control Techniques for Carbon Monoxide Emissions. U.S. Environmental
Protection Agency, EPA 452/3-79-006.
ICAC. 1997. White Paper “Selective Catalytic Reduction (SCR) Control of NOx Emissions.”
Loeffler, Dietrich, and Flatt. 1988. Dust Collection with Bag Filters and Envelope Filters.
Bertelsman Publishing Group, Braunshweig, Germany.
Pratapas, J. and J. Bluestein. 1994. Natural Gas Reburn: Cost Effective NOx Control. Power
Engineering, May 1994.
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APPENDIX A
EPA’S DRAFT PAPER CLEARING UP
THE RULE EFFECTIVENESS
CONFUSION
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Clearing Up The Rule Effectiveness Confusion
Introduction
Since its formation, EPA has been implementing rules and regulations that require states
to reduce the amount of pollution being emitted into the atmosphere. Achieving the air quality
anticipated by implementing a particular rule has not always been successful despite imposition
of numerous emission controls. In 1987 EPA acknowledged that existing air quality regulations
were not resulting in sufficient emission reductions to reach acceptable levels of air quality. The
November 24, 1987 Federal Register said “The EPA believes that one reason ozone levels have
not declined as much as expected is that reductions from national and local control measures
have not been as high as expected.”(1) This Federal Register further stated that “the
effectiveness (i.e., the ratio of actual reductions to expected reductions expressed as a percentage)
of some rules is much lower than 100 percent.” To correct or compensate for the lower than
anticipated amount of reductions, the Federal Register notice stated that “for both new and
existing rules, EPA proposes to allow States to assume not more than 80% of full effectiveness
unless adequate higher levels are adequately demonstrated.” Said another way, “we don’t believe
your rule will get as much reduction as you think it will.” This under-performance can result
from:
* some sources not implementing (or not implementing all the time) controls required by
the rule,
* some sources not installing sufficient control equipment to achieve required emission
rate,
* some sources operating installed control equipment at less than rated control efficiency,
* new source being introduced into the local area covered by the rule.
Any of these situations could result in attainment year emissions being higher than anticipated.
Even though an individual source’s emission rate is reduced to that specified in a state rule, the
overall reduction within the state may not be as great because of the above considerations.
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The 1987 Federal Register (1) defines “effectiveness” as:
Actual Reductions
effectiveness = ------------------------Expected Reductions
(1)
For complete compliance to occur, effectiveness must equal 100%. This Federal Register
recognizes however, that effectiveness is usually not 100%. To adjust for non-compliance, the
Federal Register limits the amount of reduction that a state can anticipate. This forces policy
planners to account for less than complete compliance. For example, if an agency implemented a
rule to reduce emissions by 100t/y (expected reduction), the Federal Register suggests that the
actual reduction will not be as great as the expected reduction (Equation 1). For the 100t/y goal
to be met (i.e., “effectiveness” to be 100%), the actual reduction in Equation 1 must be modified
as follows:
Reduction target * (Empirical Factor)
effectiveness = ----------------------------------------------Expected Reduction
(2)
where:
Expected Reduction = Emission reduction required as estimated by
modeling to meet air quality standard
In this example, equation 2 becomes:
Reduction target * 0.8
100% = ------------------------------------100
Solving for Reduction target: Reduction target = 125t/y
Policy makers then develop control strategies based on this Reduction target value. If an agency
implements a rule to reduce emissions by 100t/y, the policy makers must target a 125t/y
reduction to be able to achieve the needed 100t/y. Note that the results of equation 2 do not
reflect the accuracy of the emission estimates, but only adjust for the past history of complying
with a new rule.
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The 1992 Federal Register (2) defines rule effectiveness as:
Actual reduction
Rule Effectiveness (RE) = ----------------------------(3)
Expected reduction
where:
Actual reduction = (base year emissions) - (current year emission estimates)
In equation 3, the new term “RE” is an indicator that compares the amount of actual emission
reduction to the expected reduction. This metric is useful to decision makers as they evaluate
how well their policies are achieving the intended goals or how effective the rule is in achieving
expected reductions. For example, assume an agency modeling exercise indicated that 100t/y
reduction is needed in 10 years to be able to reach attainment status. Also assume the base year
inventory is 200t/y. If a 50t/y reduction is achieved 5 years into the implementation period, then
the RE = (200 - 150)/100 = 50%. At the end of 10 years, if the entire 100t/y has been removed,
then the RE = (200 - 100)/100 = 100%.
Introducing the factors contained in these equations acknowledges the reality that, in an
imperfect world, a rule intended to reduce emissions and improve air quality does not always
work as planned. Equation 2 offers, for planning purposes, an empirical solution to this problem
while Equation 3 measures the effectiveness of the solution after controls are implemented. The
empirical approach assumes that only 80 percent (or higher if an agency can substantiate) of the
required control will be achieved. To offset this shortfall, additional controls are needed. This
concept was further supported in the April 16, 1992 Federal Register(2). Under III(A)(2)(a)(2) it
is stated that “one hundred percent rule effectiveness is the ability of a regulatory program to
achieve all the emission reductions at all sources at all times.” The “extra” controls in Equation
2 compensate for parts of the air quality strategy that are not completely implemented “at all of
the sources all of the time”.
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As the air quality control community became more sophisticated, it realized that other
causes could be contributing to the inability to reach acceptable air quality levels. Two areas of
concern are the accuracy of air quality model predictions (air quality modeling issues will not be
addressed in this discussion) and the accuracy of the emission inventory accounting process
(quantity of emissions represented in the inventory). Policy makers use emission estimates to
help develop new rules that will cause the removal of a specified quantity of pollutant. They
assume that removing this amount of pollutant will lead to acceptable air quality. The amount to
be removed is usually selected as a result of various air quality modeling exercises. If the initial
quantity of emissions used in the model calculations is incorrect, then the amount of pollutant to
be reduced, as calculated by the model, may also be incorrect.
To offset an assumed underestimate of emissions, states are required to apply a
compensation factor to facility control device efficiency values. This action has the effect of
reducing the assumed efficiency of the control device (a reasonable assumption since control
equipment may fail, be off line due to equipment maintenance, and process upsets occur) and
increasing individual source emission estimates. This factor, also called Rule Effectiveness, has
a default value of 80 percent.
Very few sources measure their emissions directly using Continuous Emission Monitors
(CEM). Uncontrolled emissions at sources not monitored by CEMs are estimated using the
following equation:
emissions = emission factor * activity data
(4)
If RE is used, the equation to calculate emissions from a facility containing a control device
becomes:
emissions = emission factor * activity data * (1 - CE * RE)
where: CE = manufacturer stated control efficiency
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(5)
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The definition of RE in Equations 3 and in Equation 5 are very different. Equation 3
provides policy makers with a method to measure the amount of reduction at a point in time and
judge the success of a particular rule. Equation 5 adjusts individual facility estimates to
compensate for assessment techniques that do not account for all emissions. Even though the
philosophy behind the emission adjustments is different in each case, the same term - RE, is
used for both situations.
Why Confusion Exists
In 1992, EPA issued “Guidelines for Estimating and Applying Rule Effectiveness for
Ozone/CO State Implementation Plan Base Year Inventories.”(3) Under section 1.2 the
document states “The appropriate method for determining and using RE depends upon the
purpose for the determination: compliance program or inventory. RE discussed outside the
particular purpose may be generically referred to as control effectiveness. The following three
common uses for a control effectiveness estimate have historically been called rule effectiveness:
* Identifying and addressing weakness in control strategies and regulations related to
compliance and enforcement activities (more accurately call Compliance Effectiveness)
* Defining or redefining the control strategy necessary to achieve the required emissions
reductions designated in the CAAA (more accurately called Program or SIP Design
Effectiveness)
* Improving the accuracy or representativeness of emission estimates across a
nonattainment area (hereafter called Rule Effectiveness)”(3)
“The inventory RE is an adjustment to estimated emissions data to account for the
emissions underestimates due to compliance failures and the inability of most inventory
techniques to include these failures in an emission estimate. The RE adjustment accounts for
known underestimates due to noncompliance with existing rules, control equipment downtime or
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operating problems and process upsets. The result is a better estimate of expected emission
reductions and control measure effectiveness in future years”.(3)
Previous paragraphs provide definitions of Compliance effectiveness and Rule
effectiveness and try to make a distinction between the two. Despite these distinctions, the
second sentence of the preceding paragraph inadvisely combines concepts of both rule
noncompliance and the problem of overestimating collection efficiency of control equipment.
Even though there is a recognition that the two situations are different, the RE term is used
interchangeably in each of these examples.
Rule Effectiveness Guidance: Integration of Inventory, Compliance, and Assessment
Applications(4) was issued in January 1994. In the Introduction, the document states that “Rule
Effectiveness (RE) is a generic term for identifying and estimating the uncertainty in emission
estimates caused by failures and uncertainties in emission control programs. It is a measure of
the extent to which a rule actually achieves its desired emission reductions.” Implying a second
definition, the Introduction further states that “rule effectiveness accounts for identifiable
emission underestimates due to factors including noncompliance with existing rules, control
equipment downtime, operating and maintenance problems, and upsets.” As was previously
noted, the RE term is again used in different contexts within the same section of the same
document.
This Guidance document(4) contributes further to the confusion by using apparently
different definitions of rule effectiveness. The Glossary defines Rule Effectiveness as “a generic
term for identifying and estimating the uncertainties in emission estimates caused by failures and
uncertainties in emission control programs. Literally, it is the extent to which a rule achieves the
desired emission reductions.”
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Based on past history it is understandable that, over time, the inventory community has
used RE to describe different situations and often interchanging the definitions during the same
discussion. The RE definition has evolved, taking on slightly different meanings, depending on
the group using the term and the program to which it is being applied. Confusion results because
the inventory community often uses the term RE without indicating the context in which it is
being applied. Mangat, in a paper(5) presented at an emission inventory conference in 1992 and
in a subsequent EIIP paper (6), recognized that dissimilar definitions were being used and tried to
explain the differences.
Solutions to the Confusion
RE is currently being used to describe and solve unrelated problems. In one case it is
being used to address the failure of control equipment to operate at its stated efficiency for 100%
of the time. In the second case RE is being used to address the failure of people to implement a
rule with the required vigor.
Applying an adjustment factor is a valid approach in each of these situations.
Unfortunately, the same term (RE) is used to describe and address both cases. The inventory
community does not need more jargon. However, a solution to the current dilemma is to
abandon the RE name and replace it with two distinctive terms, each describing specifically the
situation in which it applies. Separate definitions should allow those interested in measuring
how well a rule is achieving its intended reductions to determine those results. Those interested
in adjusting actual emission estimates to compensate for upsets, downtime, etc could also meet
their needs. Each new term is described below.
The Practical Compliance Index (PCI) is to be used by those in policy positions to
measure how well a rule is achieving its intended results. The PCI is a measure of the extent to
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which a rule actually accomplishes its desired emission reductions. For example, if a new rule
has a PCI of 80%, it has caused 80% of the needed emission reductions to occur. A 100% of the
expected reductions did not (has not) occurred because not all facilities implemented controls
mandated by the rule, some facilities did not control at the emission rate required by the rule, or
unanticipated growth occurred in the area. Additionally, policy makers using historical PCI
values can develop realistic control strategies for their area.
The Operational Adjustment Factor (OAF) is to be used to adjust control efficiency
ratings of control devices. Adjustments are necessary due to control equipment down time,
subpar control device operations, and process upsets. Current methods of estimating emissions
do not account for these situations. The OAF will not be used to adjust emission factors, activity
data, or direct measurement of emissions.
How to Apply a PCI and an OAF
PCI
Air quality modeling is performed to support new rule development. Models are run to
determine how much pollutant should be removed from the air to reach an acceptable ambient
air quality concentration level. When the new rule is implemented, a strategy is developed, based
on model results, that describes the sources to be controlled and the acceptable emission rate of
each source.
The Practical Compliance Index (PCI) provides policy makers with two tools. The Index
measures how well the control strategy is progressing toward reaching the air quality goal. The
PCI is calculated by:
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PCI =
(Base year emission estimate) - (Current year emission estimate)
-----------------------------------------------------------------------------Expected reduction
(6)
The PCI measures progress toward meeting the new emission target in the designated attainment
year. PCI can be calculated periodically to provide policy makers with information on how the
policy is being implemented and the extent of compliance with new control requirements.
Past experience has shown that, even if after a new rule is fully implemented, the ambient
air quality level still exceeds the standard. One reason for this failure is lack of compliance with
a new rule. Policy makers can use this information to increase the likelihood that future emission
targets will be met. This can be done by using an empirically derived factor that is used to adjust
Equation 6. Even though the air quality modeling indicates a certain number of tons of pollutant
are needed to be removed to reach the standard, practical experience shows that, without
additional emphasis, this target will not be reached. The compensation factor in equation 6a
offsets this lack of compliance. If the goal is to achieve a 100% PCI, then equation 6 becomes:
PCI =
Reduction target * Compensation factor
-----------------------------------------------------------Expected reduction
(6a)
Where: Compensation factor has a default value of 80%
The denominator is the amount of reduction necessary, as calculated by air quality
modeling, to achieve acceptable ambient air pollutant levels. By setting the PCI to 1 (100%) and
solving for the Reduction target in the numerator, policy makers will know how much pollutant
reduction should be targeted for their control strategies. The compensation factor is analogous to
the definition of RE in equation 3. Guidance currently being used to calculate a RE factor can be
used to estimate the compensation factor in equation 6a.
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OAF
An inventory is composed of data that are used to estimate emissions. It contains
information on control efficiencies of the devices connected to the processes being inventoried.
Actual emissions are estimated either from direct measurements of the source or from
calculations using variables contained in the inventory. The most common approach to
estimating emissions is to select an emission factor associated with a process and combine it with
the activity (thruput) of the operations. This amount is adjusted by the control efficiency of the
devices attached to the process. The final product is an estimate of pollutant emitted to the
atmosphere. Actual emissions are calculated by:
Actual emissions = (emission factor) * (activity data) * (1 - control efficiency)
(7)
There are several inaccuracies associated with this approach. Even though the precision
of the emission factor or activity estimate may be poor, there is usually no quantifiable bias
associated with these values. However, because of operational process upsets, down time of the
control device, and maintenance of the control equipment, overall control efficiency of the
devices attached to the process is not as great as stated by the manufacturers. This introduces a
bias into the emission estimating process that is known qualitatively, but is not accounted for in
the inventory.
Equation 7 assumes there is no bias in the emission factor or activity data and that the
control device operates at 100 percent of its design efficiency all the time the process is running.
To reflect reality, control efficiency should be adjusted for process upsets and control device
downtime. Equation 7 then becomes:
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Actual emissions = (emission factor)unctl * (activity data) * (1 - control efficiency * OAF) (8)
Tons by-passing control device (t/y)
where: OAF = 1 - ------------------------------------------------------------------[Tons collected (t/y)] + [Tons by-passing control device (t/y)]
The OAF is determined by examining operating records for a control device or family of
devices. The amount of time it is operating, the number of process upsets, and the quantity of
pollutant that bypasses the control device during these periods can be used to create the OAF.
Recently, some emission rates are being combined with process control efficiencies to
form an emission factor that consists of a process-control device combination. Equation 8a is
used when the emission factor incorporates control efficiency.
Actual emissions = (emission factor)ctl * (activity data) * (1/CE - OAF)
(8a)
Summary
The emission inventory community has been using RE for almost a decade. Even though
the term has been used interchangeably in totally different applications, the distinctions have
been poorly understood. New terminology proposed in this paper should correct this problem.
The PCI measures the degree to which a rule is being implemented (by measuring the amount of
actual reduction and comparing it to the expected reduction). It is based on historical results
from past rule implementation efforts or from recent surveys that indicate the degree of
compliance to be expected. The PCI compensates for the failure of people to fully implement a
rule.
The OAF is a function of control equipment efficiency, the adequacy of equipment
maintenance, equipment reliability, and the stability of a process. This information is available
from records maintained at each facility. The OAF compensates for the failure of equipment to
perform at its stated capacity.
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Next Steps
- determine how this proposed approach affects existing data
- determine how existing guidance must be changed to reflect new approach
- decide what to do about previously reported data that has RE applied
- develop new guidance explaining use of PCI and OAF.
References
(1) Federal Register, Vol 52, No. 226, Tuesday, November 24, 1987, p45059
(2) Federal Register, Vol 57, No. 74, Part III, Thursday, April 16, 1992
(3) “Guidelines for Estimating and Applying Rule Effectiveness for ozone/CO State
Implementation Plan Base year Inventories”, November 1992, EPA-452/R-92-010
(4) ”Rule Effectiveness Guidance: Integration of Inventory, Compliance, and Assessment
Applications”, January 1994, EPA-452/4-94-001
(5) “Developing Present and Future Year Emissions Inventories Using Rule Effectiveness
Factors”, presented at the International Conference and course, Emission Inventory Issues,
Durham, NC, October 1992.
(6) “Emission Inventories and Proper Use of Rule Effectiveness”,
http://www.epa.gov/ttn/chief/eiip/pointsrc.htm, draft report, October 1998.
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APPENDIX B
EIIP’S TECHNICAL PAPER EMISSION
INVENTORIES AND PROPER USE OF
RULE EFFECTIVENESS
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EMISSION INVENTORIES AND
PROPER USE OF RULE
EFFECTIVENESS
Prepared by:
Emission Inventory Improvement Program
Point Sources Committee
September 23, 1998
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PURPOSE
The purpose of this document is to discuss Rule Effectiveness (RE) and explore its applicability
in a base year and a projected year emission inventory development process. This document also
presents how RE can be built into an electronic database to develop inventories.
BACKGROUND
Emission inventories for criteria pollutants are required by state and federal statutes. They have
many uses including developing control strategies to reduce emissions. Inventory data (current
and projected) are also used in air quality models that attempt to relate emissions in the inventory
to the ground-level pollutant concentrations recorded by instruments. To design effective control
strategies, inventories showing actual emissions for the period of concern are required.
Over the years, inventories have shown emission reductions due to adopted rules (regulations)
but air quality measurement data have not shown corresponding reductions in pollutant levels.
Therefore, the U.S. Environmental Protection Agency (EPA) concluded that the calculated
emissions reported in inventories were too low because the level of control efficiencies applied to
the calculations was too high.
EPA assumed that the emission inventory preparers were using the level of controls specified by
the rules and were not giving any consideration to less than full compliance. EPA thus
developed a solution to lower the level of control by multiplying the control efficiency by a
correction factor called Rule Effectiveness (RE). When no better information was available,
EPA guidance suggested a default 80% RE value. This guidance has been implemented
inconsistently by the states with unintended consequences.
Inventories reporting actual emissions are in fact only estimates of those emissions. EPA had
concerns that the emission factors for pollutant sources and abatement devices provided in AP-42
underestimate emissions because these factors do not account for equipment malfunctions and
abatement device downtime. Emissions could also be underestimated due to ignorance of rules
or circumvention of controls, process upsets, spills, and other day-to-day operating parameters.
These parameters can significantly affect the estimates of actual emissions. Many of these
parameters apply to all pollutant sources and not only to sources subject to rules. RE can not and
should not be applied to uncontrolled sources. Therefore, the use of correction factors to account
for these problems and unknown parameters in the emission calculation procedure is very
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appealing. There is a need for guidance to improve emission calculation procedures to account
for the parameters mentioned above. These issues will be addressed in another document titled
Effects of Source Operational Problems, being prepared by the Point Sources Committee of the
Emission Inventory Improvement Program (EIIP).
However, RE plays an important role in emission inventory and rule development. Rules are
adopted to reduce emissions by specified amounts. RE is a function of actual emissions and the
emissions estimates calculated using limits specified in a rule. Actual estimated emissions
should take into account various operational problems such as equipment malfunction and
abatement device downtime. RE measures how well emission controls called for by a rule are
being achieved in the real world. Therefore, RE measures the degree to which the actual
estimated emissions approaches the expected emissions called for by a rule. If the actual
estimated emissions are equal to the expected emissions based on rule limits specified in a rule,
then the effectiveness for that rule is 100%. If the actual estimated emissions are higher than the
expected emissions based on rule limits, then the RE is less than 100%. If the actual estimated
emissions are lower than the expected emissions based on rule limits, then the RE is greater than
100% (i.e., over compliance).
BASE YEAR EMISSION
CALCULATIONS AND RE
The purpose of adopting and implementing rules is to reduce emissions. Emissions can be
reduced by lowering a source’s activity, or the emission factor that represents that activity, or
adding control devices. Permit conditions are sometimes imposed on a facility to lower or limit
activity levels. There are two ways to lower emission factors (final rate of emissions) for a
source:
The processes can be modified to inherently low emissions rates; or,
Pollution control equipment can be incorporated into the process.
In either case, emissions estimated before and after regulatory changes will show actual emission
changes due to controls. The actual estimated emissions when compared with the expected
emissions based on rule limits will give the RE.
The final controlled emission factor =
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uncontrolled emission factor x (1 - % reduction
specified in the rule/100 x % RE /100)
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Actual estimated emissions from sources subject to a rule can be calculated using two different
methods. The first method uses actual data, and the second uses values stated in the rule(s) to
estimate the actual emissions after rule implementation. These two methods will be described in
detail in the subsections below and will show that when actual data are used, RE is not part of the
emission calculations. When actual data are not available, estimated RE is used to calculate
emissions.
Estimating Emissions Using Actual Data: Emission calculations from point sources usually
fall in this category because detailed source and throughput information is usually collected and
stored in a database. One emissions calculation procedure for a process is to multiply the activity
(throughput) by an emission factor, and by the control equipment efficiency if control equipment
is used. The emissions can be updated by updating throughput, as long as control equipment has
not changed. Emission factors are obtained from source tests, AP-42, engineering estimates, or
other sources. If continuous monitoring is available, emission factors can be calculated and
stored in the database. Source-specific emission factors, when available, are preferred to
generalized factors. Emission factors are not derived from any rule, rather they represent best
estimates of actual factors for the source and should account for any operational problems. The
selection of emission factors for use in an inventory should be left to the estimator’s judgment.
Guidance in selecting emission factors is also available in EIIP documents where preferred
emission estimation methods are recommended.
The control equipment efficiency used can be either specific or general. The control efficiency
for actual emission calculations should be based on design specifications, testing, or estimated
values for the equipment. The control efficiency selected should be adjusted to reflect actual
conditions. If a given type of equipment has problems operating continuously, design values
should be adjusted to reflect the problems. This gives the estimated actual emissions. When a
general control efficiency is used, it is not based on the design specification but is a “best
judgment” value that takes into account deterioration, maintenance needs, and other day-to-day
issues encountered. Emissions thus calculated are not the same as emissions estimated using
values in rules, but represent actual estimates for emission inventories.
Emissions from some area source categories also can be calculated using actual data. For
example, using the EIIP preferred method, emissions from the architectural coatings category can
be calculated by obtaining the coating usage data and the solvent content for each type of coating
by surveys.
Note that RE is not used in emission calculation procedures using actual data. RE can be
calculated from the actual estimated emissions and emissions calculated using limits in the rule.
For rulemaking or for emission inventories (by source categories), there may be some value in
estimating RE for individual sources; emissions from all sources subject to a rule should be
aggregated by categories. The composite emission factor obtained by dividing the total actual
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emissions by the total throughput when compared with the rate of emissions specified in the rule
will give the estimated RE. Consistent units must be used in these calculations and comparisons.
RE can also be obtained using the emissions instead of the emission factors as discussed above.
Estimating Emissions Using Information from Rules: Emissions from area source categories
are sometimes calculated using emission rate data from rules. Emissions for area sources are
usually calculated by categories instead of by individual sources and are based on estimated
emission factors and activity data. Activity levels are usually derived from available surrogate
data, and the emission factors selected generally will change over the years. If there are changes
in a category (e.g., new technology or operational changes), then the emission factors should be
reevaluated. The activity usually changes with changes in the surrogates used.
When a rule is adopted and implemented, new emission factors to account for the controls are
usually not developed. Lower emissions are accounted for in the control efficiency. It is usually
not easy to determine the percentage of control called for by a rule, but the emission inventory
estimators must estimate the level of controls called for by a rule. It is important to remember
that the emissions are estimated for a category and not for individual sources.
For example, emissions from underground gasoline storage tanks can be calculated by adjusting
the uncontrolled emission factor by the percent control specified in the rule and the estimated
RE. This is not the best or the preferred method, but is commonly used. This method can be
used to calculate emissions from, for example, the architectural coatings category because the
final controlled emission factor(s) are estimated using controls specified in the rule instead of
obtaining actual emission factors independently.
EMISSION PROJECTIONS
The estimated RE value obtained in the base year calculations can be used to develop a future
year RE value. The base year inventory, as discussed above, may or may not include RE in
emission calculations. To prepare projections for categories subject to rules with future
implementation dates, the estimated net controls for the rule (controls specified in the rules and
the estimated RE) are required. Some sources in the category may comply before the rule’s
specified implementation date and others may comply after the implementation date. Also, as
time passes, more sources comply and the RE increases. Therefore, changing the RE by specific
dates will result in better estimates for future year emissions. Future year emissions for
categories subject to rules are calculated by the following equation:
Future year emissions = base year emissions x growth factors x control factors
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The control factors represent the net controls, based on the controls specified in the rule and the
anticipated RE. Similarly, historical emissions can be recalculated using the above equation.
EMISSION INVENTORY SYSTEM
An emission calculation system using RE data is shown in this section. This system works for
calculating future and historical emissions for all point and area sources and also for base year
calculations for area source categories. In this system, two separate files are used to estimate the
net emission factors for any given year, an emission factor file and an emission control file. Only
the area source categories will have emission factors in the emission factor file, and only the
categories subject to rules will have control information in the emission control file as described
below.
Emission Factor File: Data in this file are organized by categories and are only for area sources.
Categories with major point sources will not have any entry in this file because their emissions
are calculated source-by-source using actual data. Only the uncontrolled emission factors for
area sources (with known generation dates) are kept in this file. Changes in emission factors
over the years due to reasons other than rulemaking (e.g., changes in technology) should be
reflected in this file. Therefore, more than one record can exist for a given category with
different emission factors and their corresponding effective dates.
Emission Control File: Data in this file are for categories subject to a rule. This file contains
data for point as well as area source categories. Each record will have a unique identification
number for each category, the percentage of reduction of the pollutant (maximum achievable)
specified by the rule, and the percent RE (% RE) as shown in Table 1. A category can have more
than one record to represent different dates with changing % RE as shown in Table 1, as well as
to represent different geographic areas of rule applicability.
TABLE 1
EXAMPLE OF EMISSION CONTROL FILE
Category
Identification
% Control
Date
Pollutant
% RE
10
60
Jan 1, 1998
VOC
20
10
60
Jan 1, 1999
VOC
70
10
60
Jan 1, 2000
VOC
95
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This file can be expanded to store additional information about the rule if desired (date of
adoption, rule description, etc.). The example in Table 1 is for category identification number 10
that is subject to a rule requiring 60% VOC control. The percent RE improves from 20% to 95%
over 3 years and assumes there is no rule requiring control prior to January 1, 1998.
Emission Calculation: For the emission calculation procedure, the latest emission factor should
be selected and used for all years after this date except when data in the emission control file
exist. In such a case, the net emission factor is calculated by multiplying the emission factor(s)
from the emission factor file and the control(s) and the effectiveness from the emission control
file. With this method, past and future emissions can be calculated if throughput data (or growth
rates) are available for various years.
Base year emissions for area source categories are similarly calculated by multiplying the base
year throughput with the appropriate emission factors from the same files. A simple system of
files can be set up for the data requirements stated above. For a more elaborate system, refer to
the paper Developing Present and Future Emissions Inventories Using Rule Effectiveness
Factors by T. Mangat, T. Story, and T. Perardi. This paper was presented at the October 1992
Emission Inventory Conference sponsored by the Air & Waste Management Association in
Durham, North Carolina.
Rule Penetration: When a rule for a source category is adopted with a stated level of control,
some sources may be exempted or may have limits set at higher levels (lower level of control).
This can lower the overall control achieved for a source category subject to a rule. Frequently,
some sources in a category will not comply or will be late in complying with the limits in the
rule. All of these factors should be considered when determining the RE for a source category.
It simplifies calculations if the maximum control achieved from the rule is kept constant and only
the RE is varied to indicate the effectiveness of controls. The net control for the source category
is then obtained by multiplying the percentage of control and the RE. There is no need to store a
separate rule penetration factor in the database.
RECOMMENDATIONS
It is desirable to know the level of controls and the RE being achieved from rules. If the RE is
determined and found to be lower than 100%, measures such as stepping up enforcement can be
taken to correct the deficiencies. Knowing the actual controls (controls x RE) achieved in the
base year will help in estimating future controls from the rule. Similarly, base year level of
control when backtracked correctly should yield historical control information. RE should be
tracked at a source category level. When base year emissions are estimated using actual data, RE
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CHAPTER 12 - CONTROL DEVICES
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and the controls specified in the rules are not required for calculating actual estimated emissions.
Actual estimated emissions can also be directly calculated from source test data or by mass
balance. To calculate actual estimated emissions from point sources, consideration should be
given to uncertainties associated with various factors that affect emissions. Many area source
categories may use controls specified in rules and the RE to calculate base year emissions.
For forecasting emissions, all source categories subject to a rule should track the controls
specified by the rule and the RE. The estimated RE should account for rule penetration if the
source category contains sources exempt from the rule. This adjustment to RE to account for
rule penetration is not needed if the exempt sources are grouped under a different source category
showing no controls.
Can RE be used in calculating emission inventories? The answer is yes, and when used
correctly, it can help in calculating actual estimated emissions--especially in forecasts and
backcasts. Is it important to know the RE for a rule? The answer is yes, even if it is difficult to
calculate or estimate. To evaluate the effects of a promulgated rule, emission controls specified
by the rule and the RE should be estimated.
12.B-8
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CHAPTER 12 - CONTROL DEVICES
APPENDIX C
CROSS REFERENCE OF AIR
POLLUTION CONTROL DEVICE NAMES
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CHAPTER 12 - CONTROL DEVICES
AIR POLLUTION CONTROL DEVICE NAMES
Some control devices are known and referred to by more than one name. For example, a fabric
filter is also referred to as a baghouse. To assist readers in correlating the name of a specific
control device of interest where the name is not one of those used in this document to the name
of the device as used in this document, a list of cross reference names is provided in the
following table. The Control Device column in the table presents the different names used for
specific control devices. For each name in the Control Device column, the name for the device
that is used in this document is provided on the same row in the Cross Reference column. For
example, where “baghouse” is presented in the Control Device column, the Cross Reference
column lists “fabric filter”, which is the name used in this document.
EIIP Volume II
12.C-1
CHAPTER 12 - CONTROL DEVICES
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APPENDIX C
CROSS REFERENCE OF AIR POLLUTION CONTROL DEVICE NAMES
Control Device
Absorbers (Scrubbers)
Absorption
Ammonia Injection
Annular Orifice Venturi Scrubber
Baghouse
Butadiene Adsorber
Catalytic Afterburners
Centrifugal Collector
Centrifugal Cyclone
Cyclone/Fabric Filter
Cyclones
Dry Cyclones
Dry Scrubbers
Dry Sorbent Injection
Dry Sorbent Scrubber
Dual Cyclones
Duct Injection
ESP
FGR
Fuel Cell Incineration
Impingement Scrubbers
Incineration
LEA
LNB
Multiple cyclones in series
Multiple cyclones
Multiclones
NSCR
OFA
Reburn
SCR
SNCR
Sodium Scrubbers
SOFA
12.C-2
Cross Reference Name
Wet acid gas scrubber
Wet acid gas scrubber
Selective Noncatalytic Reduction (SNCR)
Wet PM scrubber
Fabric filter
Carbon adsorber
Catalytic Incinerator
Mechanical Collector
Mechanical Collector
Mechanical Collector/Fabric Filter
Mechanical Collector
Mechanical Collector
Spray Dryer Absorber
Dry Injection
Spray Dryer Absorber
Mechanical Collector
Dry Injection
Electrostatic Precipitator
Flue Gas Recirculation
Thermal Incinerator
Wet PM Scrubber
Thermal Incinerator
Low Excess Air
Low NOx Burner (LNB)
Mechanical Collector
Mechanical Collector
Mechanical Collector
Nonselective Catalytic Reduction (NSCR)
Over-Fire Air
Natural Gas Burners/Reburn
Selective Catalytic Reduction (SCR)
Selective Noncatalytic Reduction (SNCR)
Spray Dryer Absorber
Staged Overfired Air
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CHAPTER 12 - CONTROL DEVICES
APPENDIX C
CONTINUED
Control Device
Spray Drying
Thermal Afterburners
Thermal Oxidation
Venturi Scrubbers
Wet FGD
Wet Scrubbers
Urea Injection
Staged Combustion for Gas Turbines
EIIP Volume II
Cross Reference Name
Spray Dryer Absorber
Thermal Incinerator
Thermal Incinerator
Wet PM Scrubber
Wet Acid Gas Scrubber
Wet Acid Gas Scrubber, or Wet PM Scrubber
Selection Noncatalytic Reduction (SNCR)
Dry-Low NOx (DLN), Dry-Low Emissions
(DLE), or SoLo NOx
12.C-3
CHAPTER 12 - CONTROL DEVICES
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APPENDIX D
DATA COMPILATION FOR SECTION 3
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CHAPTER 12 - CONTROL DEVICES
DESCRIPTION OF HOW THE DATA WERE COMPILED
D.1 CONTROL EFFICIENCIES TABLE
Individual table included for each pollutant of interest-CO, NOx, PM, SOx, VOC.
Tables present control efficiencies (CE) for selected control devices for each
unique combination of emission source and control device reported in the
references. Where one or more references report an average CE, or CE range, for
the same combination of emission source and control device, the references are
examined to determine which one provides the best quality data and the data from
that reference are selected and shown in the table. (The “best” data are
determined using professional experience and judgment.)
Each pollutant table contains columns that show an average CE and CE range for
each unique combination of emission source and control device for which data are
provided in the references.
EIIP Volume II
-
The emission source is identified in the Process and Operation columns.
For example, where a reference reported a CE for a boiler burning coal,
“Fuel Combustion-Coal” is shown in the Process column and “Boiler” is
shown in the Operation column.
-
A single column (Control Device Type) is used to identify the type of
control device. The description of the control device provided in the
reference was used to assign a control device type.
-
The average CE is presented in the column labeled Average CE (%). The
reference citation for the average CE is shown in the column to the right
labeled Reference. Two columns are used to show a range with the lower
value on the left in the column labeled CE Range (%) Minimum and the
upper value on the right in the column labeled Maximum. The reference
citation for the range is shown in the column to the right labeled
Reference. It should be noted that the average CE and CE range could be
from two separate references. Where data were not available, the column
is empty.
-
Where only a minimum value for the CE was available from the
references, the value is presented in the Minimum column. The Maximum
column is left empty. Where only a maximum value was available, the
12.D-1
CHAPTER 12 - CONTROL DEVICES
7/14/00
value is presented in the Maximum column and the Minimum column is
left empty.
-
Where one reference provided only one value of a range (minimum or
maximum) and a second reference provided both values (minimum and
maximum), the data from the second reference are presented in the table.
The data from the first reference were not used.
D.2 DATA FOR CONTROL DEVICES NOT EVALUATED
12.D-2
Table D-1 presents control efficiencies (CE) reported in the references for control
devices not evaluated in this document.
Individual table included for each pollutant of interest-CO, NOx, PM, SOx, VOC.
-
Each pollutant table contains columns that show an average CE and CE
range by emission source and control device for which data are provided in
the references.
-
The reference citation for each row of data in the table is indicated in the
Reference column.
-
The emission source is identified in the Process and Operation columns.
For example, where a reference reported a CE for a boiler burning coal,
“Fuel Combustion-Coal” is shown in the Process column and “Boiler” is
shown in the Operation column.
-
The description of the control device provided in the reference is included
in the Control Device Description column.
-
The description of the control device provided in the reference was used to
assign a control device type which appears in the Control Device Type
Column.
-
Where a reference provided an average CE, the CE is shown in the
Average CE (%) column. Where a reference provided a minimum or
maximum CE, or both, they are shown in the CE Range (%) Minimum and
Maximum columns, respectively. Because the data in the table are “as
entered” and have not been evaluated, an Average CE and a range CE
obtained from the same reference will appear in two separate rows.
EIIP Volume II
7/14/00
EIIP Volume II
TABLE D-1
CONTROL EFFICIENCIES FOR CONTROL DEVICES NOT EVALUATED IN THIS DOCUMENT
CE Range (%)
Pollutant
Process
Operation
Control Device
Type
Wood Industry
Dryer/Press Exhaust
Biofilter
NOX
Fuel Combustion- Coal
Boiler
Limestone Injection
Multi-stage Burner
NOX
Fuel Combustion- Coal
Boiler
WAS-SNOX
NOX
Fuel Combustion- Coal
Boiler
GR-SI
NOX
Petroleum Industry
Process Heaters
Natural Air Lances
NOX
Petroleum Industry
Process Heaters
Forced Air Lances
NOX
Wood Industry
Dryer/Press Exhaust
Biofilter
PM
Aluminum Industry
Baking Furnaces
PM
Fuel Combustion- Wood
Boiler
Fabric Filter Wth
Reduction Cell
Granular-bed
Moving Filter
PM
Fuel Combustion- Wood
Boiler
PM
Fuel Combustion- Wood
Boiler
Granular-bed
Moving Filter with
Electrostatic
Precipitator
Gravel Bed Filter
95
Minimum Maximum
Reference
Value
Value
Reference
30
50
EPA, 1995
50
60
EPA,
1992b
90
EPA,
1992b
70
EPA,
1992b
10
20
EPA,
1992b
50
60
EPA,
1992b
80
95
EPA, 1995
99
EPA, 1995
EPA, 1995
90
95
98
99.2
AWMA,
1992
AWMA,
1992
12.D-3
CHAPTER 12 - CONTROL DEVICES
CO
Average CE
(%)a
CONTROL EFFICIENCIES FOR CONTROL DEVICE NOT EVALUATED IN THIS DOCUMENT (CONTINUED)
CE Range (%)
Control Device
Type
Gravel Bed Filter
Gravel Bed Filter
Average CE
(%)a
95
95
Pollutant
Process
PM
Fuel Combustion- Wood
PM
Fuel Combustion- Wood
Operation
Boiler
Boiler
PM
PM
General
Metallurgical Industry
General
Core Separator
Waste Heate Boiler Tubular Cooler
SOX
Fuel Combustion- Coal
Boiler
Duct Injection
SOX
Fuel Combustion- Coal
Boiler
Furnace Injection
SOX
Fuel Combustion- Coal
Boiler
SOX
Fuel Combustion-Coal
Boiler
Limestone Injection
Multi-stage Burner
NOXSO
90
SOX
Fuel Combustion- Coal
Boiler
WAS-SNOX
95
SOX
Fuel Combustion- Oil
Boiler
Duct Injection
SOX
Fuel Combustion- Oil
Boiler
Furnace Injection
SOX
Metallurgical Industry
Lead Smelters
DMA Absorber
VOC
Aluminum Industry
Baking Furnaces
VOC
Chemical Manufacturing
SOCMI Reactor
Fabric Filter with
Reduction Cell
Condenser
Minimum Maximum
Reference
Value
Value
Reference
EPA, 1995
AWMA,
1992
95
98
EPA, 1998
70
80
AWMA,
1992
25
>50
AWMA,
1992
25
50
AWMA,
1992
50
60
EPA,
1992b
EPA,
1992b
EPA,
1992b
25
>50
AWMA,
1992
25
50
AWMA,
1992
92
95
AWMA,
1992
99
EPA, 1995
50
95
CHAPTER 12 - CONTROL DEVICES
12.D-4
TABLE D-1
7/14/00
EIIP Volume II
EPA,
1992a
7/14/00
EIIP Volume II
TABLE D-1
CONTROL EFFICIENCIES FOR CONTROL DEVICES NOT EVALUATED IN THIS DOCUMENT (CONTINUED)
CE Range (%)
Pollutant
Process
Operation
Control Device
Type
Degreasing- Cold Cleaner
General
Hot Vapor Recycle
VOC
Degreasing- In-line Cleaner
General
VOC
Degreasing- Open Top Vapor
Cleaner
Idling Losses
VOC
Degreasing- Open Top Vapor
Cleaner
Idling Losses
Above-Freezing
Freeboard
Refrigeration
Below-Freezing
Freeboard
Refrigeration
Increased Freeboard
Ratio
VOC
Degreasing- Open Top Vapor
Cleaner
Working Losses
VOC
Degreasing- Open Top Vapor
Cleaner
Working Losses
VOC
Degreasing- Open Top Vapor
Cleaner
Working Losses
VOC
Degreasing- Open Top Vapor
Cleaner
VOC
Minimum Maximum
Reference
Value
Value
Reference
62
69
AWMA,
1992
61
AWMA,
1992
12.D-5
11
58
AWMA,
1992
27
47
Above-Freezing
Freeboard
Refrigeration
Below-Freezing
Freeboard
Refrigeration
Increased Freeboard
Ratio
18
50
AWMA,
1992
AWMA,
1992
26
54
Working Losses
Refrigerated Primary
Condenser
18
Fabric Coating
General
VOC
Food Industry
Fermentor
VOC
Food Industry
Smokehouses
Inert Gas
Condensation System
Scrubber, Wet with
Biofilter
Scrubber, Vortex
51
EPA, 1995
VOC
Gasoline Marketing
Loading
Submerged Filling
60
AWMA,
1992
VOC
Gasoline Marketing
Service Stations
90
VOC
Gasoline Marketing
Service Stations
Vapor Balancing
Stage I
Vapor Balancing
Stage II
25
99
52
AWMA,
1992
AWMA,
1992
AWMA,
1992
EPA, 1992a
90
AWMA,
1992
95
EPA, 1995
AWMA,
1992
AWMA,
1992
CHAPTER 12 - CONTROL DEVICES
VOC
Average CE
(%)a
CONTROL EFFICIENCIES FOR CONTROL DEVICES NOT EVALUATED IN THIS DOCUMENT (CONTINUED)
CE Range (%)
Pollutant
VOC
General
Process
Operation
General
Control Device
Type
Condenser
VOC
General
VOC
Magnetic Tape Manufacture
Natural Gas
Processing
Drying Ovens
VOC
Surface Coating
Polymeric Coating Vapor Recovery
VOC
Surface Coating
VOC
Wood Industry
Vinyl
Coating/Primer
Dryer/Press
Exhaust
Average CE
(%)a
90
Reference
EPA, 1991
Vapor Recovery
Condenser
95
95
Minimum Maximum
Value
Value
Reference
50
95
EPA,
1992a
98
EPA,
1992a
CHAPTER 12 - CONTROL DEVICES
12.D-6
TABLE D-1
AWMA,
1992
AWMA,
1992
Vapor Recovery
90
Biofilter
70
90
EPA,
1992a
EPA, 1995
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CHAPTER 12 - CONTROL DEVICES
APPENDIX E
EXAMPLE CALCULATIONS
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CHAPTER 12 - CONTROL DEVICES
Example E-1--Coke and Coal Fired Boilers
For this example, consider a coal-fired boiler that operates as follows:
Throughput:
1,764 MMBTU/hr
Operating Hours:
8,500 hr/yr
Permitted emissions: 0.03 lb particulate/MMBTU
Control Device:
Electrostatic precipitator
Assumed CE:
99 percent (from Table 12.3-6)
If this process and its associated control equipment operated exactly as designed for
the entire 8,500 hr/yr, the expected emissions would be:
expected annual
particulate emissions
= 1,764 MMBTU/hr x 8,500 hrs/yr x 0.03 lb
particulate/MMBTU
= ~450,000 lbs/yr
= ~225 tons/yr
However, low voltage or other malfunctions might cause the ESP to occasionally
operate at 95 percent efficiency rather than 99 percent efficiency. During such events,
the emission rate would be 0.12 lb particulate/MMBTU (four times the “normal”
emission rate of 0.03 lb particulate/MMBTU). If these anomalous conditions occurred
during 5 percent of the total operating hours (i.e., 425 of the 8,500 hrs per year) for the
plant, annual particulate emissions would be:
actual annual
particulate emissions
= 1,764 MMBTU/hr x (8,500 hrs/yr x 95%) x
0.03 lb/MMBTU
+ 1,764 MMBTU/hr x (8,500 hrs/yr x 5%) x
0.12 lb/MMBTU
= 517,293
= 258 tpy
Thus, in this example, a 4 percent reduction in ESP efficiency for 5 percent of the operating time
would increase actual annual particulate emissions by 33 tpy (20 percent) over the permitted
amount. If the state’s emission inventory estimate for this facility is based only on the permitted
figures, the 33 tpy (20 percent) actual increase for this facility would be missed.
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CHAPTER 12 - CONTROL DEVICES
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Example E-2--Wood Products Dryer/Press
For this example, consider a wood products dryer/press that operates as follows:
Operating schedule:
7,920 hr/yr
Process exhaust flow rate: 8,000 dscf/min
Permitted emissions:
0.01 gr/dscf
Control Device:
Fabric filter
Permitted emissions:
Assumed CE:
27.4 lb/hr VOC (based on NSPS)
94 percent wet ESP and thermal oxidizer,
If this process and its associated control equipment meet the emission limits when
operated normally and the process and control equipment operate normally for the
entire 7,920 hr/year, emissions would be:
Expected annual PM10 emissions
= 0.01 gr/dscf x 8,000 dscf/min x 60 min/hr x
1 lb/700 0 gr
= 0.686 lb/hr
= 0.686 lb/hr x 7920 hr/yr
= 5430 lb/yr
= 2.71 tpy
Expected annual VOC emissions
= 27.4 lb/hr x 7,920 hrs/yr
= 217,008 lb/yr
= 108.5 tpy
In this example, if the thermal oxidizer were to fail for 4 hours per month, resulting in a
reduction from 94 percent efficiency to 50 percent efficiency for VOC, VOC emissions will
increase by 113.3 tpy, or an increase of 4.4 percent over expected annual emissions.
Occasional bag wear and tear will cause the fabric filter to malfunction. The malfunction of a
fabric filter is immediately apparent as it results in accumulation of particles in the vicinity of the
device, as uncontrolled gas escapes through holes in the fabric. Emissions resulting from the
malfunction can be estimated by collecting and weighing the amount of dust escaping through
the filter.
12.E-2
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CHAPTER 12 - CONTROL DEVICES
APPENDIX F
EXAMPLE ANNUAL EMISSION
INCREASES FOR VARIOUS SCENARIOS
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CHAPTER 12 - CONTROL DEVICES
A general formula for calculating increases in annual emissions due to malfunctioning control
devices is:
I
=
ta x (CEn - CEa) / (100% - CEn)
I
CEn
CEa
=
=
=
ta
=
Increase in annual emissions due to a malfunctioning control device (%)
Normal control efficiency (%)
Malfunction control efficiency (%) [note: use the actual control
efficiency. Do not express as a percent of the normal control efficiency.]
Operating time under malfunction conditions (% of total hours)
where:
The three examples in this appendix use the above formula to calculate annual emission increases
for three hypothetical examples. In each example, we assume a specific malfunction efficiency
(e.g., assume that a malfunctioning fabric filter operates at 97.5 percent efficiency) and show the
annual emission increases that would result under different combinations of design efficiencies
and percentage malfunction time.
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12.F-1
CHAPTER 12 - CONTROL DEVICES
EXAMPLE F-1:
7/14/00
VERY HIGH DESIGN EFFICIENCY AND SLIGHT DECREASES
IN ACTUAL EFFICIENCY RESULT IN SIGNIFICANT ANNUAL
EMISSION INCREASES
Consider a hypothetical ESP that operates under 97.5 percent efficiency during a minor
malfunction. Table F-1 shows the emission increases that would occur if the device operated
under malfunction conditions from 1 to 10 percent of the time, and if the ESP was otherwise
expected to operate at design efficiencies between 98 and 99.5 percent.
For example, if the control device design efficiency is 99.5 percent, and the control device
operates under malfunction conditions (at 97.5 percent efficiency) for 5 percent of the time, the
increased emissions due to the malfunction would add 20 percent to the expected annual
emission. The data in Table F-1 are presented graphically in Figure F-1.
As you can see in the example of Table F-1, small decreases in the control percentage can result
in large percentage increases in actual emissions if the design efficiency is high.
Table F-1. Percentage Increase Over Expected Annual Emissions for an ESP Operating at
97.5% Efficiency During Malfunction
Percentage Downtime
Design
Efficiency
99.5%
1%
2%
4.0%
8.0%
99.0%
1.5%
98.5%
98.0%
12.F-2
3%
4%
5%
6%
7%
8%
9%
10%
12.0% 16.0% 20.0%
24.0%
28.0%
32.0%
36.0%
40.0%
3.0%
4.5%
6.0%
7.5%
9.0%
10.5%
12.0%
13.5%
15.0%
0.7%
1.3%
2.0%
2.7%
3.3%
4.0%
4.7%
5.3%
6.0%
6.7%
0.3%
0.5%
0.8%
1.0%
1.3%
1.5%
1.8%
2.0%
2.3%
2.5%
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CHAPTER 12 - CONTROL DEVICES
100.0%
90.0%
80.0%
70.0%
60.0%
Percent Increase
50.0%
10%
9%
8%
7%
6%
Percentage of Tim e
5%
Operating Under
Malfunctioning
4%
Conditions
3%
40.0%
30.0%
20.0%
10.0%
2%
0.0%
99.5%
1%
99.0%
98.5%
98.0%
Design Efficiency
Figure F-1. Percent Increase in Actual Annual Emissions with Malfunction Efficiency at
97.5%
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12.F-3
CHAPTER 12 - CONTROL DEVICES
EXAMPLE F-2:
7/14/00
LOW DESIGN EFFICIENCY AND LARGE DECREASES IN
ACTUAL EFFICIENCY RESULT IN LESS SIGNIFICANT
EMISSION INCREASES
In contrast to the case in Example F-1, emission changes are less significant if the design
efficiency is low, as might be the case with a NOx scrubber designed to operate at control
efficiencies between 80 - 70 percent. If, for example, a NOx scrubber with a design efficiency of
80 percent actually operated at 50 percent efficiency during malfunction conditions 5 percent of
the year, actual annual emissions would only be 7.5 percent over the expected annual emissions.
Table F-2 and Figure F-2 show the percentage increase for various scenarios in which the NOx
scrubber operates at 50 percent control efficiency during malfunction.
Table F-2. Percentage Increase Over Expected Actual Emissions for NOx Scrubber
Operating at 50% Efficiency During Malfunction
Percentage Downtime
Design
Efficiency
1%
2%
3%
4%
5%
6%
80.0%
1.5%
3.0%
4.5%
6.0%
7.5%
9.0%
79.0%
1.4%
2.8%
4.1%
5.5%
6.9%
78.0%
1.3%
2.5%
3.8%
5.1%
77.0%
1.2%
2.3%
3.5%
76.0%
1.1%
2.2%
75.0%
1.0%
74.0%
9%
10%
10.5% 12.0%
13.5%
15.0%
8.3%
9.7%
11.0%
12.4%
13.8%
6.4%
7.6%
8.9%
10.2%
11.5%
12.7%
4.7%
5.9%
7.0%
8.2%
9.4%
10.6%
11.7%
3.3%
4.3%
5.4%
6.5%
7.6%
8.7%
9.8%
10.8%
2.0%
3.0%
4.0%
5.0%
6.0%
7.0%
8.0%
9.0%
10.0%
0.9%
1.8%
2.8%
3.7%
4.6%
5.5%
6.5%
7.4%
8.3%
9.2%
73.0%
0.9%
1.7%
2.6%
3.4%
4.3%
5.1%
6.0%
6.8%
7.7%
8.5%
72.0%
0.8%
1.6%
2.4%
3.1%
3.9%
4.7%
5.5%
6.3%
7.1%
7.9%
70.0%
0.7%
1.3%
2.0%
2.7%
3.3%
4.0%
4.7%
5.3%
6.0%
6.7%
12.F-4
7%
8%
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CHAPTER 12 - CONTROL DEVICES
100.0%
90.0%
80.0%
70.0%
60.0%
Percent Increase
50.0%
40.0%
30.0%
9%
20.0%
7%
10.0%
5%
72.0%
73.0%
1%
Percentage of Tim e
Operating Under
Malfunctioning
Conditions
70.0%
Design Efficiency
74.0%
75.0%
76.0%
77.0%
3%
78.0%
79.0%
80.0%
0.0%
Figure F-2. Percentage Increase in Actual Annual Emissions with Malfunction Efficiency
at 50%
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12.F-5
CHAPTER 12 - CONTROL DEVICES
EXAMPLE F-3:
7/14/00
MODERATELY HIGH DESIGN EFFICIENCY AND LARGE
DECREASES IN ACTUAL EFFICIENCY RESULT IN VERY HIGH
INCREASES IN ACTUAL ANNUAL EMISSIONS
As would be expected, there will be very high increases in actual annual emissions if the design
efficiency is high and the actual efficiency greatly decreases for even a short while. Failure of a
VOC control system (e.g., due to flame out or pump failure) can result in large efficiency drops
that may go undetected if the VOC is odorless or colorless, or if the stack does not vent near
people. For example, if a malfunctioning control device operates at only 25 percent efficiency
for 1 percent of the year, but is supposed to operate at 95 percent efficiency year round, the
annual emissions will increase by 14 percent. If the control device is supposed to operate at
99 percent efficiency year round, the annual emissions will increase by 74 percent!
Table F-3 and Figure F-3 show the percentage increase for various scenarios in which a
malfunctioning control device operates at only 25 percent control efficiency.
Table F-3. Percentage increase Over Expected Annual Emissions for VOC Adsorber
Operating at 25% Efficiency During Malfunction
Design
Efficiency
Percentage Downtime
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
99.0%
74.0% 148.0
222.0
296.0
370.0% 444.0% 518.0%
592.0% 666.0% 740.0%
98.0%
36.5% 73.0% 109.5
146.0
182.5% 219.0% 255.5%
292.0% 328.5% 365.0%
97.0%
24.0% 48.0% 72.0% 96.0% 120.0% 144.0% 168.0%
192.0% 216.0% 240.0%
96.0%
17.8% 35.5% 53.3% 71.0%
88.7%
106.5% 124.3%
142.0% 159.8% 177.5%
95.0%
14.0% 28.0% 42.0% 56.0%
70.0%
84.0%
98.0%
112.0% 126.0% 140.0%
94.0%
11.5% 23.0% 34.5% 46.0%
57.5%
69.0%
80.5%
92.0%
103.5% 115.0%
93.0%
9.7%
19.4% 29.1% 38.9%
48.6%
58.3%
68.0%
77.7%
87.4%
97.1%
92.0%
8.4%
16.8% 25.1% 33.5%
41.9%
50.3%
58.6%
67.0%
75.4%
83.8%
91.0%
7.3%
14.7% 22.0% 29.3%
36.7%
44.0%
51.3%
58.7%
66.0%
73.3%
90.0%
6.5%
13.0% 19.5% 26.0%
32.5%
39.0%
45.5%
52.0%
58.5%
65.0%
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800.0%
700.0%
600.0%
500.0%
Pe r ce n t Incr e as e
400.0%
300.0%
200.0%
9%
7%
100.0%
5%
91.0%
90.0%
De s ig n Efficie n cy
1%
92.0%
93.0%
94.0%
95.0%
96.0%
97.0%
3%
98.0%
99.0%
0.0%
Pe r ce n tage o f T im e
Op e r atin g Un de r
M alfun ctio ning
Co nd ition s
Figure F-3. Percentage Increase in Actual Annual Emissions with Malfunction Efficiency
at 25%
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APPENDIX G
DATA SOURCES FOR SECTION 5
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CHAPTER 12 - CONTROL DEVICES
Data Needed
Target Control Level or Emission
Rate
Data Source
NSPS, MACT, NESHAP, or other
Federal regulations
State regulations
Permit Conditions
Actual Control Level of Emission
Rates
Comments
Regulations and permit conditions
may list the target control level or
emission rate.
These values may be included in
the (RACT/BACT/LAER
Clearinghouse) accessible via the
EPA website.
EIIP guidance
The EIIP guidance provides
calculation methods and expected
control levels for numerous source
categories.
NSPS, MACT, NESHAP, or other
Federal regulations guidance on
maximum downtime
Regulations and permit conditions
may indicate maximum allowable
downtimes or maximum allowable
excess emissions that are below the
target control rate but still within
compliance with the rule.
State regulations for maximum
downtime
Permit conditions for maximum
downtime
EIIP Volume II
Quarterly CEM reports where
required by Federal or state rules
(e.g., for utility boilers)
Some sources (e.g., utility boilers)
must file quarterly CEM reports
with the state agency. Data in these
reports might not make it to the
emission inventory branch unless
you specifically request them.
EIIP guidance
The EIIP guidance provides
calculation methods and expected
control levels for numerous source
categories.
Facility reports for excess emissions
Facilities may file reports of excess
emissions with state permitting or
compliance staff, particularly if
permit conditions require.
State databases (e.g., DEERS in
South Carolina) for tracking excess
emissions
Some states (e.g., Texas and South
Carolina) keep databases of excess
emissions reported by facilities.
State compliance or permitting staff
State compliance and permitting
staff will be the best sources of
information regarding the expected
amount of downtime and the
expected degree of reduced control.
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